322 MOLLUSC A. 



special portions of the coelom, and as such are nearly always in 

 free communication with other parts of the coelom. In Nautilus 

 alone are they isolated. The glandular tissue of the excretory 

 organs is arranged in various ways in different forms (for which 

 see accounts of separate groups). The forms in which the nitro- 

 genous waste is got rid of is said to be guanin in Cephalopods, 

 uric acid in many Opisthobranchs, and urea in Lamellibranchs. 



The pericardial or viscero-pericardial division of the coelom also 

 often contains glandular tissue, wlych constitutes the pericardial 

 gland. 



Reproduction is always sexual, and no cases of parthenogenesis 

 are known in the phylum. The power of reproducing lost parts, 

 whether cast off voluntarily or lost accidentally, is considerable 

 parts of the foot and its appendages, siphons of some bivalves, dorsal 

 papillae, etc., of some Nudihranchs, cephalic tentacles, arms of 

 Cephalopoda; and very often the part reproduced may bear organs 

 of a complicated kind, e.g., eyes, suckers, etc. But this power of 

 reproducing lost parts is never so great so far as is known as to 

 lead to the complete formation of a new individual from the part 

 removed from the body, i.e., asexual reproduction is unknown in 

 the group. The hermaphrodite condition is fairly common. 



The genital glands, however much they may be modified, are 

 always to be regarded as portions of the coelom. This fact is 

 perfectly obvious in the Scaphopoda, and in some Lamellibranchs 

 and Gastropoda, in which they open into the renal division of the 

 coelom; and in the Solenogastres and Cephalopoda, in which they 

 open into the peri visceral part of the coelom. But in the majority 

 of Lamellibranchs and Gastropods it is not so obvious ; but there 

 can be no doubt of this being the real, if modified and concealed, 

 relation, when such forms are compared with others in the same class 

 in which the two organs communicate ; and further, it must not be 

 forgotten that in some Lamellibranchs, and in Paludina amongst 

 Gastropods, the genital cells can, in their origin in the embryo, be 

 directly related to the coelom. 



Development. We cannot here enter into details of the develop- 

 ment ; the greatest variety prevails from the large meroblastic eggs 

 of Cephalopoda, through viviparous forms such as Paludina, to 

 the immense majority of forms in which the eggs are small, and 

 the young hatched at an early stage as a larva. The larva always 

 has the trochosphere form, and soon acquires a shell-gland on its 

 dorsal, and a rudiment of the foot on its ventral surface (Fig. 268). 



