MOLLUSCS. 



251 



Lungs, which are cavities of the iiiaiith' lined witli respiratory folds, occur only in 

 the pulmonate gasteropods, where they will be described at length. 



The renal organs, nephridia, or organs of Bojanus as they are frecpiently called 

 from the celebrated anatomist who 



discovered them, are always present. 

 They are usually symmetrically dis- 

 posed, there being one on each side 

 of the body. Each nephridium con- 

 sists of a tube, the inner poition of 

 which communicates with a portion 

 of the body cavity, while the other 

 opens externally. In the interior 

 portion are well-developed glands, fiu. 257.— Ncpiiiui 



' . ■ \ ,■, , ternal opening; 



which excrete uric acid, while tlie giamiuiar port 



nil of Unio; (j^ glandular portion; n ex- 

 o, opening between {p) pericardium and 

 1 of nephridium; r, rei)r(Kluctive orifice; t, 



^ . . ..v^.i-glaudular portion of nephridium; V, ventricle. 



outer, non-glandular portion is merely 



an afferent duct. That these nephridia are homologous with the segmental organs of 

 worms is more than possible, and the ])robability is strengthened by the fact that their 

 internal openings are ciliated, and that in many forms they serve for the extrusion of 

 the seminal, as well as for excretory, products. 



Reproduction is here always a sexual operation, fission and budding being un- 

 known. As a rule the two sexes are combined in the same individual, but numerous 

 marine gasteropods, and all cephalopods, are dioecious. The sexual glands are placed 

 on either side of the body, and either i)]>t'n through duets of their own, or by means 

 of the nephridia, as mentioned above. 



Ill all except the cephalopods there is a more or less complicated metamorphosis 

 in ])assing from the egg to the adult. According to the amount of food-yolk, the seg- 

 mentation is regular or irreguhir, the result Ix'ing a morula or inulberiy-like mass. 

 Soon a portion invaginates, just as we ni;iy juish in one side of a rubber ball, or, owing 

 to the presence of a great quantity of food-yolk, this process may be obscured. The 

 result, however, is in both cases the formation of a two-layered sac, the gastrula. 

 The mouth of the gastrula, the blasto])ore, soon closes more or less coni])letely, and 

 from the middle portion is developed the foot, while the two ends correspond respec- 

 tively with the mouth and vent. Occasionally one of these openings persists, but not 

 infrequently a new invagination takes jilace to form the openings, the injuishing of the 

 integument being always within the limits of the blasto])ore. From the outer layer of 

 the gastrula is developed the epidermal structures of the boily, while the inner gives 

 rise to the middle division of the digestive tract. From this inner division cells are 

 also budded off between the two layers, forming the mesoblastic tissues, and later one 

 or more spaces appear in this mesoblast, the body cavity. Further details of the 

 internal development may be found in special works, but for our purposes we need to 

 follow the changes in external form a little further. 



At about the time of the invagination. ;i portion of the (Kiter surface devflops ;i 

 circle of long hairs or cilia. This circle, which is known as the velum, embraces only 

 a small portion of the exterior, and since both mouth and anus, when formed, are 

 behind it, it follows that the area so circumscribed is pre-oral. Not infreipiently a 

 single longer hair or flagellum occupies the centre of the velar area, marking the differ- 

 entiation of the ectodermal layer into nervous tissue, the future supra-oesojihageal 

 ganglia. This stage is the trochosphere, and jiresents a close resemblance to the larva 



