440 



CRUSTACEA. 



Cf. 



jaws can be distinguished from the region of the body (thorax) bearing 

 the biranious appendages, but there is no distinct boundary between 

 these two regions. The anus is situated at the extremity of the 

 small stump-like abdomen, which succeeds the thorax and is often 

 only indicated by two caudal appendages. Posterior antenna? are in- 

 variably absent, while the anterior pair persists, even in the adult, as 

 small organs of attachment. The oral apparatus is situated on a 

 ventral prominence of the cephalic region, and consists of an upper 

 lip with palps, two mandibles and four inaxillse, of which the two 

 last unite to form a sort of under lip. On the thorax there are 



usually six pairs of many-jointed 

 biramous appendages, the elongated 

 cirriforrn rami of which are richly 

 beset with hairs and seta3 and 

 serve to set up currents in the 

 water in which the particles of food 

 are brought to the animal. The 

 stump -shaped abdomen bears an 

 elongated cirrus, which is bent to- 

 wards the ventral surface between 

 the thoracic appendages, and con- 

 stitutes the male copulatory organ. 

 There are numerous and very pecu- 

 liar variations in the shape of the 

 whole body. Not only may the de- 

 position of calcareous matter in the 

 mantle be wanting, and the bira- 

 mous thoracic appendages be reduced 

 in number or even absent, but the 

 mouth parts and the appendages 

 may also be lost (Peltogastridce), 

 and the body may be reduced to 

 the form of an unsegmented tube, 

 sac, or lobed disc. 



Nervous system and sense 

 organs. The Cirripedia possess a paired cerebral ganglion and a 

 ventral chain of ganglia, of which there are usually five pairs, but 

 which are sometimes fused to a common ganglion mass (Balanidce). 

 There is a double eye, which, although rudimentary, corresponds 

 to the unpaired Nauplius eye. 



An alimentary canal is absent only in the Rhiza/cephaki. In the 



Fio. 349. The organization of Lepas, 

 after removal of the integument. 

 Cd, Cement gland and duct ; L, 

 liver ; T, testis ; Vd, vas deferens ; Or, 

 ovary ; Od, oviduct ; Cf, thoracic 

 appendages. Other letters as in 

 fig. 34S. 



