304 



COMPARATIVE ANATOMY 



CHAP. 



FIG. 206. Case of Balanus 

 Hameri, from the side (after Dar- 

 win), s, Scutum ; t, tergum. 



appendage, the male copulatory organ (p). This is bent forwards on the ventral side, 

 and lies between the tendril-like feet. The thorax consists of 6 indistinct segments. 

 In Lepas we find, as already mentioned, 5 shell pieces or calcareous plates of the 

 integumental fold (mantle), one unpaired and four paired. The unpaired piece (c) lies 

 on the dorsal side and is called the carina. The paired pieces (s, t) lie to the right 

 and left ; the anterior are called the scuta, the posterior the terga. The cleft in the 

 mantle or shell lies posteriorly and ventrally. Accessory shell pieces, impaired as 

 well as paired, may be found in addition to the above. 



In the Balanidce (Figs. 206 and 207), in contradistinction to the Lepadidce, the 

 attached anterior portion of the body is not prolonged 

 like a stalk. The mantle forms by calcification several 

 strongly united shell pieces which surround the body 

 like a rampart, to which the scuta and terga form a 

 kind of movable lid. 



In the Abdominalia, which live in the shells of 

 Cirripedes and Molluscs, the number of the thoracic 

 segments is reduced, and the body is enclosed in a 

 flask-shaped mantle which does not calcify, as the 

 shell of the host affords sufficient protection. The 

 Apoda live parasitically, like the Abdominalia, in 

 the mantle of other Cirripedes. The mantle fold 

 does not here attain development. The body of 11 

 segments assumes the form of a fly -maggot, hav- 

 ing lost the tendril-like feet. The Crustacean body suffers the greatest degree of 

 degradation in the Rhizocephala, which live parasitically on the abdomen of 

 Decapoda. In this case we 



find only an unsegmented v" \ "\ o 



sac (Fig. 208), entirely devoid 

 of limbs, containing the 

 viscera (testes, germarium, 

 cement glands, ganglion), 

 and itself enveloped in 

 another outer sac-like mem- 

 brane. This outer membrane 

 which surrounds the brood 

 cavity has been considered, 

 erroneously it appears, as the 

 mantle. An aperture in it 

 leading to the exterior is 

 termed the cloaca. The body 

 is attached to that of the 

 host by means of a short 

 peduncle. On this stalk 

 of attachment arise long 

 branched filaments which 

 penetrate the body of the 

 host and conduct nourish- 

 ment from its body to the 



or; 



FIG. 207. Balanus tintinabulum, after removal of the 

 right half of the calcareous ring, o-o, Edges of the aperture 

 of the ring sk ; sc, scutum ; t, tergum ; aj, anterior (adhering) 

 antennae ; ov, ovarium ; ovi, oviduct ; wo, female genital aper- 

 ture ; m, muscles for moving the scuta and terga ; ad, musculus 

 adductor scutorum (after Darwin). 



parasite in a manner similar to that in which the roots of a plant convey nourishment 

 out of the earth. 



The Rhizocephala are classed as a special order of the Entomostraca (Kentrogo- 

 nidce). It is from thfeir ontogeny alone that we learn that they are Crustaceans at 



