290 LOWER INVERTEBRATES. 



appendages. The peculiar eyes of Onchidium and the chitons will be mentioned 

 further on. At the first glance the eyes of the gasteropods (and also those of the 

 cephalopods) seem strikingly like those of the vertebrates. They have an external 

 cornea behind which is a lens, a vitreous humor, and lastly a retina containing rods 

 and cones. A great difference exists in the fact that these rods and cones are on the 

 front side of the retina, and between them and the optic nerve is the pigment layer. 

 Frequently these organs are seated upon stalks or tentacles which are capable of 

 retraction. In our common garden snail may be found an example of this structure, 

 and the retraction is accomplished by the drawing in of the end of the tentacle in the 

 same way that one inverts the finger of a glove. 



The sexes of the gasteropods may be separate or combined in one individual, and 

 copulatory organs ai'e frequently present. Most forms lay eggs, a few, however, bring 

 forth living young, the eggs undergoing development within the parent. The eggs, 

 which are numerous, are frequently enveloped in capsules, differing greatly in foi'm 

 and ornamentation, each capsule containing a number of eggs. We shall note the 

 appearance of some of the more interesting of these egg-cases in connection with 

 the forms to which they belong. 



In the development of the eggs of the Cephalophora there is much diversity in the 

 early stages, according as the amount of food-yolk present is large or small. This 

 also influences the character of the gastrula which is formed, and the entoderm may 

 either be solid or in the form of a sac. Soon after the invagination, the outer surface 

 almost always becomes covered with cilia in certain regions, the most prominent being 

 a ring around the body in advance of the mouth. This is the velum, so characteristic 

 of molluscs. At about the same time a small portion of the outer la^er sinks in to 

 form the gland which eventually secretes the shell, and the foot begins to appear. 

 These changes all take place within the egg, and upon the development of the 



cilia, the larva begins to turn round and round, thus 

 rendering it difiicult for the student to obtain a dis- 

 tinct view of what is going on inside the embryo, 

 or even to draw the external appearance. The shell 

 gland begins to secrete the shell, which arises first 

 as a small plate, but soon takes the form of a cap 

 enveloping the posterior part of the body and 

 then gradually acquires a spii-al form. The region 

 of the velum also exhibits a change. Instead of 

 Vio.S22^eV^^Eoiisdwersa. ^eing a ring around the body it becomes a two- 



lobed plate fringed with cilia, which serve as loco- 

 motory organs after the young has hatched. 



From this point the development is usually direct, no startling metamorphoses 

 being introduced. The velum almost always disappears, and the body and shell 

 gradually acquire their adult Structure. Still the variations and the changes under- 

 gone throw considerable light on the classification and arrangement of the different 

 forms. 



The classification of the odontophorous molluscs is still in an uncertain condition, 

 notwithstanding the fact that they have been so extensively studied. In fact, there 

 are scarcely two authors who agree as to the rank and relationship of the different 

 associations of forms. This difference of opinion is partly due to the varying impor- 

 tance accorded to the different characters,' and partly to the fact that a linear arrange- 



