380 



COMPARATIVE ANATOMY 



CHAP. 



In the Cirripcdla the eggs are concealed in the interior of the shell between the 

 mantle and the body of the animal. In the PJiizocciihala the integument splits into 

 an outer lamella (mantle) and an inner lamella (wall of the visceral sac). Between 

 the two a brood cavity arises (Fig. 248, p. 373), into which the eggs, emerging 

 from the female atrium, enter, and in which they develop. The eggs are enclosed 

 in a richly branched sac, formed of a chitinous membrane, and exactly repeating 

 the form of the cement glands which enter the female atrium. The sac is in 

 reality nothing but the inner cuticular lining of the cement glands which is ejected 

 when the eggs are laid and becomes filled with the eggs as they leave the ovary. 

 The Xauplii which develop out of the eggs in the brood cavity reach the exterior by 

 means of its aperture called the cloaca. In the Branchiopoda there are various arrange- 

 ments for the care of the brood. In the shelled forms, the eggs are concealed under 

 the shell, either in appendages of certain pairs of limbs which are transformed into ovi- 

 sacs or brood pouches (Apus), or on filamentous appendages of such limbs (Es(Jicritfir). 

 In the Cladocera the eggs develop in a brood cavity (Fig. 192, p. 289), which forms 

 dorsally between the shell and the body, becomes entirely closed towards the exterior 

 by special arrangements, and contains a fluid for the further 

 nourishing of the brood. In some Cladoccra, a saddle-shaped 

 thickening of the dorsal integument of the shell (Ephippium) 

 covers every one or two winter eggs, and is cast off with the eggs 

 as a protection during winter. This ephippium is often provided 

 with adaptations which facilitate its passive distribution in 

 space. In the Copepoda, the eggs which emerge from the 

 genital apertures reach the interior of ovisacs which stand out 

 freely from the body (in the genital segment), and which are 

 formed from the secretion yielded by the cement glands. 

 Where the two genital apertures lie somewhat far apart later- 

 ally or dorsally in the double genital segment there is a pair 

 of ovisacs ; where they lie very near each other on the ventral 

 side, one unpaired median ovisac is formed (Fig. 194, p. 290). 

 These ovisacs are so characteristic of the Copepoda, that by this 

 means the most deformed parasitic Copepodan females may be 

 recognised (Fig. 255). In the Notodcfphydcc alone the eggs pass 

 into a brood cavity enclosed by the integumental folds. The 

 female of the Leptostraca shelters the eggs and hatched larva 

 between the lamellated thoracic feet. In the female of the 

 Arthrostraea, Schizopoda, and Cumacca the brood lamellte on 

 the basal joints of the thoracic feet already described develop 

 at the approach of sexual maturity. These brood lamellre, by 

 locking into one another from right and left, form the base 

 of a brood cavity, whose cover is the ventral (sternal) integu- 

 ment of the thorax (Fig. 218, p. 318). The eggs reach this 

 brood cavity and develop in it. The hatched young or larv?e 

 often stay some time in it. The females of the Decapoda attach 



I'J 



FIG. '2ii[>. Lernae- 

 ocera esocina, female. 

 iia, Frontal eye ; tj, U, 



1 3 , t t rudimentary thor- h em ; t the r i eopo d a by means of the secretion 



acic feet ; d, intestine ; . . 



od oviduct ; es, egg * the cement glands already mentioned on tne under side ot 



sacs ; A, arm processes the abdomen. In the Brachyura, whose shield-shaped abdomen 



at the anterior end of j s bent round on the sternal side of the cephalo-thorax, the 



the body (after Clans). abdomen is generally decidedly larger and broader in the 



female than in the male, and more adapted for covering and protecting the egg 



masses. The same difference, though not so pronounced, may be seen in the 

 Macrura also. 



