Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/680

<J li U 8 T A C E A tubercles ; these are the antennae and mandibles. At this immature stage of its development the egg-membrane bursts before any internal organ, or even any tissue except the cells of the cutaneous layer, is formed. The young animal may now be said to be in its nauplius-stage, but its nauplius-skin resembles more nearly a second egg-membrane within which its further development proceeds. The ten pairs of appendages of the cephalic and thoracic divisions FIG. 34. Older larva produced from Zo e a represented in fig. 33. The last seg ment and the last two pairs of feet of the middle-body are wanting. Magn. 20 diam. (Fritz Miiller.) of the body make their appearance simultaneously, and at a later period the five abdominal feet. Soon after the young Mysis has cast its nauplius envelope, it leaves the brood-pouch of its mother. In Squilla mantis the eggs do not adhere to the abdominal feet of the parent (which in this genus are tranchiferous), but are (says Fritz Miiller) deposited in the form of thin, round, yellow plates within its submarine burrow. The spawn is consequently difficult to procure, and quickly dies when removed from its natural hatching- place. In the embryo of Squilla the heart is short ; the body is long and segmented, but without appendages ; the tail is bilobate, and there are indications of the rudiments of six pairs of limbs. If it acquires more limbs before exclusion, the youngest larva must be on a par with the youngest of Euphausia observed by Glaus. Only two larval forms of Squilla are mentioned by Fritz Miiller ; the elder of these zoea (fig. 35) resembles the mature Squilla, FIG. 35. Zoe.i of a Stomapod, probably SquiUa. Magn. 15 diam. (Fritz Miiller.) particularly in the structure of the great raptorial thoracic feet and of the last cephalic pair ; but the six pairs of feet which follow these are still wanting, although their somites are clearly seen. The abdomen shows rudiments of four pairs of branchial feet and one or two pairs of biramose natatory feet, but the tail has no appendages and still appears as a simple lamina. The investigations of Goodsir, in 1843, 1 made us acquainted with a most singular family of Crustacea, the Diastylid.ee, or Cumacece, which have been placed in the Podophthalmia near to Mysis. In general aspect the adult animal presents the most larval and embryonic characters, and might with propriety have been treated as a larval form, had not Goodsir, and subsequently Krciyer, actually taken the young from the brood-pouch of the parent. The antennae are very small, the thoracic feet are, inmost, furnished with setae ; in Cuma and Alauna the abdominal segments are moniliform and destitute of appendages. The caudal segment bears two long bifurcated styles. In Bodotria (fig. 36) five of the abdominal somites Fio. 36. Male of Bodotria. Magn. 10 diam. (Fritz Miiller.) bear finlets. The young examined by Kroyer, taken from the brood-pouch of the female (which resembles that in Mysis), were already one-fourth the length of the parent, which they resembled in every respect. Whether or not there is a considerable development of the young of Cumacece within the brood-pouch of the parent is not cer tainly known. In the embryo the caudal portion is bent upwards as in the Isopoda, and the last pair of thoracic feet are wanting. The development of the Edriophthalmia, or sessile-eyed Crustacea, is more simple than that of the stalk-eyed forms. In the " rock-slater," Ligia (fig. 37), the embryo is bent upwards within the egg, as in Mysis, and has also, like Mysis, a larval membrane within which the young Ligia is developed. In Mysis this larval skin may be compared to a nauplius; in Ligia, however, it is desti tute of appendages, and resembles a maggot with a long simple tail (fig. 37). The dorsal surface of the young ! IG. 3ft. Maggot-like larva of Ligia Magn. 15 diam. R, remains of egg-mem brane. We see on the lower surface, from before backwards, the anterior and posterior antennae, the mandibles, the anterior and posterior maxillse, maxillipeds, six ambulatory feet, the last segment, of the middle-body destitute of appendages, five abdominal feet, and the caudal foot. (Fritz Muller.) Ligia is united to the larval skin a little behind the head. A foliaceous appendage is produced at this point, but exists only for a short time, and disappears before the young slater quits the brood-pouch of the mother. The young animal, when it commences to take care of itself, resembles the parent, save that it has only six, instead of seven pairs of ambulatory feet, and the last thoracic somite is but slightly developed and is destitute of appendages. The sexual peculiarities in this as in the young of other Crustacea are not developed at this early period ; thus the males lack the hand-like enlargements of the anterior ambulatory feet, and the copulatory appendages are also absent. The eggs and early stages of Asellus aquaticus have 1 Edinb. New Phil. Journ. 1843. See also Bell, British Stalk-eyed Crustacea, 1853, pp. 321-333, and Fritz Miiller, Fur Darwin, Eugl.

trans, p. 81