Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/673

CRUSTACEA G39 parts of the structure bear a close correspondence with an auditive vesicle and a tympanic membrane. 1 ORGAN OF tSMELL. Professor Owen 2 refers the sense of smell to a small sac, fringed with fine hairs, opening externally by a narrow cleft in the basal joint of the first or median antennae. A branch of the antennal nerve terminates in a small prominence at the bottom of this sac. From the presence of some minute siliceous particles within the cavity (although it is admitted that these must have found their way in from the exterior fortuitously) Dr Farre 3 has been led to suggest that the small antennse are acoustic organs, and that the grains of sand may act as otolites. Milne- Edwards admits as indubitable the presence of well-developed organs of smell, but considers we are reduced to conjecture when we are required to point out the precise seat of those organs. 4 ORGANS OP TASTE. Like almost all other animals the Crustacea select their food, showing decided preference for particular kinds; this selection is doubtless actuated by two senses, smelLand taste. Whether we are correct in assigning to the inner pair of antennae the duties of the olfactory organ or not, it cannot be doubted that the sense of taste is distributed over that portion of the tegumentary mem brane which lines the interior of the mouth and oesophagus, but there is no modification of these parts which needs to be specially noticed here. ORGANS OF NUTRITION. In the larval stages of the higher Crustacea, and also among the adult lower and simpler forms, fewer of the somites have their paired appendages differentiated to perform special offices. Thus in the larval Decapod the chief natatory organs are the maxillipeds; this is also the case in the Merostomata. In Limulus (fig. 12) all the locomotory organs are also subservient to the _Fio. 12. Underside of Limulus polyphemus, Latr. C=cephalon ; Th=thorax ; A, rudimentary abdomen ; T=telson. (The eyes 1, cannot be seen in this figure, being on the upper surface of head-shield.) 2, The chelate antennules; 3a, antennje (?a detached antenna of male); 4-7, mandibles, maxillae, and maxilliped; m, the mouth; 6, operculum, bearing on its inner and upper surface the ovaries and reproductive organs ; 9-13, branchi- gerous feet. duties of nutrition, being organs of locomotion at their distal, and mandibles and maxillae at their proximal extremity. In fact, as already stated, we have abundant evidence to 1 Spence Bate marks this organ as "olfactory" in the crab (Brit. Assoc. Reports, Bristol, 1875, pi. i. fig. 10, and explanation). Mivart calls the green gland the kidney of the lobster, and Bays "no organ of smell has been determined " (Pop. Sci. Rev., vol. vii., 1868, p. 350) See Fritz Miiller s suggestions as to this green gland, p. 652. 2 Lectures Comp. Anat. 1855, 2nd edition, p. 311. 3 Phil. Trans., 1843. 4 Milne -Edwards, in Todd s Cyclopcedia of Anatomy, vol. i. p. 768. prove that the maxillary organs of the Malacostraca are but modifications of entire limbs, translated from the locomotive series and set apart as special mouth-organs. By far the larger proportion, of the Crustacea have a proper normal mouth furnished with suitable organs .of mastication, but among the parasitic Copepoda and certain aberrant parasitic Isopods, &c., they become merely organs of attachment, tho mouth being suctorial ; or (as in the Ilhizocephala) it may be altogether wanting, and the limbs completely lost, and from the point of attachment root-like tubes may be developed, which, sinking deep into the body of the host, convey to the parasite its nutriment ready digested and prepared. If instead of these latter we examine the Decapoda we shall find the mouth placed centrally near the front and upon the under side of the cephalon. It is provided with a small simple median piece, called a labrum, or upper lip, in front, and a bifid metastoma, or lower lip, behind ; the paired appendages (mandibles, maxillae, &c.) being placed on either side of the buccal orifice. The food, whether living or dead, being first seized by the forcipated thoracic feet, is brought near to the maxillipeds, and by the help of these external organs of prehension portions are sepa rated and introduced by the maxillae to the trenchant and powerful mandibles, when having undergone further sub division they are swallowed. No organ corresponding to a tongue exists in the Crustacea, the mouth being only the anterior and outward expansion of the oesophagus, which is short, rises verti cally, and terminates directly in the stomach. The wall of the stomach is TIG. 13. Gasiric Teeth of Crab i e , i and Lobster, composed of twomembranous la> gtomach of common cral)) Cancer pagurus, laid open, showing 6, 6, 6, some of the calcareous plates inserted in its muscular coat ; g, g, the gastric teeth, which when in use are brought in contact with the sides of the smooth fixed plate m c, c, the muscular coat ; 16 and 16", the gastric teeth enlarged to show their grinding surfaces ; 2, gastric teeth of common lobster, Ho- marus vulgaris; Za and 36, two crusta cean teeth (of Ditfiyrocaris) from the Carboniferous series of .Renfrewshire. layers, separated by one of muscular fibres, which in crease in thickness at the openings leading from the O3sophagus and into the intestine. The stomach is globular in form and of great capacity, and may be divided into an anterior or " cardiac " part, and a posterior or " pyloric " region. The food 011 reaching the " cardiac " region of the stomach is subjected to a further process of mastication, by means of a complex apparatus composed of several calcareous pieces, moved by appropriate muscles, inserted in the membranous wall of the stomach (fig. 13, la), armed with a smooth median plate and two lateral molar-like-organs, Laving a singular mimetic and superficial resemblance to the molar teeth of some small marsupial rodent. Two smaller points (bicuspid in the lobster, tricuspid in the crab) complete the calcareous apparatus ; in the pylorus a series of fine hairs are placed, which, doubtless, act like a strainer, preventing the escape of the coarser particles of the food until they have repeatedly been subjected to the molar-like action of the gastric teeth. A long and straight intestine continues from the stomach backwards, and terminates beneath the telson. Two coacal salivary glands of a greenish colour are situated on either side of the oesophagus. The liver in the Decapoda is of large size, and bilaterally symmetrical : its structure is highly ramified, not solid like the human liver. The secreted fluid or bile is poured by

two openings into the pylorus.