Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/372

This page needs to be proofread.


332

BRACHIOPODA

nerves. The under arm-nerve, which lies between the dermal pits, at the base of which is one large cell which small arm-sinus and the surface, supplies nerves to the is thought to secrete the chseta, as in Chsetopods. These muscles of both arm-sinuses (1 igs. 2 and 3). Medianly, pits are not isolated, but are connected by an ectodermal it has its origin in the sub-oesophageal ganglion, which, ridge, which grows in at the margin of the mantle and like the supra-oeso- forms a continuous band somewhat resembling the ectoo7 phageal, is drawn out dermal primordium of vertebrate teeth. In spite of some assertions to the contrary, all the laterally, though not to the same extent. Brachiopods which have been carefully investigated have In the middle line been found to be male or female. Hermaphrodite forms the sub - oesophageal are unknown. Embryology.—Little real advance has been made in nerve mass is small; the ganglion is in fact our knowledge of the embryology of the Brachiopoda drawn out into two within recent years. Kovalevsky’s researches {Izv. Obshch. halves placed on Moskov. xiv. 1874) on Argiope {Megathyru) are still the either side of the ‘ most complete as regards the earlier stages. Segmentabody. From each of tion is complete, a gastrula is formed, the blastopore these sub-oesophageal closes, the archenteron gives off two coelomic sacs which, ganglia numerous as far as is known, are unaffected by the superficial nerves arise. Passing segmentation of the body that divides the larva into three from the middle line segments. The walls of these sacs give rise at an early outwards they are—- stage to muscles which enable the parts of the larva to (i.) the median pallial move actively on one another (Fig. 4, B). About this nerve to the middle stage the larvae leave the brood-pouch, which is a lateral Fig. 3.—Diagram of nervous system of Crania; of the dorsal mantle j or median cavity in the body of the female, and lead a from the dorsal side. The nerves running to /.. , amoll the dorsal parts are white, with black edges ; (11-f numerous . small free swimming life in the ocean. The anterior segment those running to the ventral parts are solid nerveS — the Circum- broadens and becomes umbrella-shaped ; it has a powerful black. Magnified ; after Blochmann. 1, cesophagus; 2, supra-oesophageal commissure; 3, OSSOpilcigtJcti oum row of cilia round the rim and smaller cilia on the general circum-oesophageal commissures; 4, underarm- mjggUj.gS—which DaSS surface. By the aid of these cilia the larva sw ims nerve ; 5, great arm-sinus ; 6, small arm-smus ; . 1 7, tentacle; 8, lip of lophophor; 9, infra- round the OeSOpnagUS actively, but owing to its minute size it covers very little (esophageal commissure; 10, chief arm-nerve ; , ,r f.i • ,r distance, and this probably accounts for the fact that 11, secondary arm-nerve; 12, nerves to ten- hO me cniei ai tacles; 13, sub - oesophageal ganglion; 14, nerve Or supra-0680- where Brachiopods occur there are, as a rule, a good dorsal lateral nerve; 15, sub - oesophageal ,,.. ,,1 i ^ • portion of the secondary arm-nerve; 16, phageaf ganglion, many in one spot. The head bears four eye-spots, and it is median palliai nerve of dorsal lobe of mantle ;

under arm- continually testing the ground (Fig. 4, A, C). The second 17, anterior occlusor muscle; 18, posterior V V , , occlusor muscle; 19, obliquus superior muscle; nerve to the lopno- segment grows downwards like a skirt surrounding the 20, levator brachii muscle. ph0r and its muscles ; third segment, which is destined to form the stalk. It bears at its rim four bundles of very pronounced chsetse. (iv.) the lateral pallial nerve to the sides of the dorsal mantle. Laterally, the sub-oesophageal ganglia give off After a certain time the larva fixes itself by its stalk to (v.) nerves to the ventral mantle, and finally they supply some stone or rock, and the skirt-like second segment (vi.) branches to the various muscles. There is a special turns forward over the head and forms the mantle. ^ hat marginal nerve running round the edge of the mantle, but goes on within the mantle is unknown, but presumably the connexion of this with the rest of the nervous system the head is absorbed. The chsetae drop off, and the lophois not clear; probably it is merely another concentration phor is believed to arise from thickenings which appear in the dorsal mantle lobe. Recently the Plankton Lxpeof the diffused sub-ectodermal nervous fibrils. The above account applies more particularly to Crania, dition has brought back, and Simroth (Ergeb. Plankton but in the main it is applicable to the other Inarticulata Expedition, ii. 1897) has described, a few. larval Brachiowhich have been investigated. In Discinisca and Lingula, pods of undetermined genera, two of which at least were however, the sub-oesophageal ganglion is not drawn out, pelagic, or at any rate taken far from the coast. These but lies medianly ; it gives off two posteriorly-directed nerves to the stalk, which in Lingida unite and form a substantial nerve. Sense organs are unknown in the adult. The histology of Brachiopods presents some peculiar and many primitive features. As a rule the cells are minute, and this has especially stood in the way of embryological research. The plexus of nerve - fibrils which underlie the ectoderm and are in places gathered up into nerves, and the great development °J; Fig_ 4._Three larva, stages of Megatnyris (Argiope). a, larva which has just left brood-pouch; B, longitudinal connective tissue, are WOltny OI section through a somewhat later stage; C, the fully-formed embryo just before fixing the neo-embry i• 1VTvir.li nf ttlif* Intfpr takes forming Beecher.segment; Highly magnified. 2, (?); second or mantle-forming notice. Much Of 4, eye-spots;1, 5,anterior set*; 6,segment; nerve mass 7, alimentary canal; 8,segment; muscles, 3, third or sta the form of ~ hyaline supporting tissue, embedded in wbicli are scattered cells and fibies. larvte, which resemble those described by Fritz Muller The lophophor and stalk are largely composed of this (Arch. Naturg. 1861-62), have their mantle turned over tissue. The ectodermal cells are large, ciliated, and their head and the larval shell well developed No stalk amongst the ciliated cells glandular cells are scattered. has been seen by Simroth or Fritz Muller, but in other The chitinous chsetse have their origin in special ecto- respects the larva resembles the stages in the development