MOLLUSCS. 



251 



Fig. 357. — NepLntUuiti of Unio; g, glandular portion; n ex- 

 ternal opening; o, opening between (p) pericardium and 

 glandular portion of nephridium; r, reproductive orifice: t, 

 non-glandular portion of nephridium; u, ventricle. 



Lungs, which are cavities of the mantle lined with respiratory folds, occur only in 

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



The renal organs, nephridia, or organs of Bojanus as they are frequently 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 portion of 

 which communicates with a portion 

 of the body cavity, while the other 

 opens externally. In the interior 

 portion are well-developed glands, 

 which excrete uric acid, while the 

 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 probability 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 open through ducts of their own, or by means 

 of the nephridia, as mentioned above. 



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

 in passing from the egg to the adult. According to the amount of food-yolk, the seg- 

 mentation is regular or irregular, the result being a morula or mulbei-ry-like mass. 

 Soon a portion invaginates, just as we may push 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 blastopore, soon closes more or less completely, and 

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

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

 infrequently a new invagination takes place to form the openings, the inpushing of the 

 integument being always within the limits of the blastopore. Fi'om the outer layer of 

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

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

 also budded oif 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, a portion of the outer surface develops a 

 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 infrequently 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 sujjra-oesophageal 

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



