304 



COMPARATIVE ANATOMY 



CHAP. 



Fig. 20S. — 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 consist.? 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, unpaired as 



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



In the Balanidm (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 Aldominalia, which live in the shells of 

 Oirripedcs 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 sufiicient protection. The 

 Apoda live parasitically, like the Ahdominalia, 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 gi-eatest degree of 



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



Dccapoda. In this case we 



find only an unsegmented 



sac (Fig. 208), entu-ely 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 



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



out of the earth. 



The Bhizocephala are classed as a special order of the Entomostraea (Kentrogo- 



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



Fig. 207.— Balanus tintinabulum, after removal of tlie 

 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). 



