128 SYSTEMATIC ZOOLOGY 



degenerate than the females, the alimentary canal often being 

 wanting. For some time it was thought that there were no males, 

 since the females are not only three or four times as large, but also 

 much more numerous. This latter fact is due to the different 

 kinds of eggs produced, and their method of development. Dur- 

 ing the summer, eggs with thin shells are formed, and of two 

 sizes, the larger developing into females and the smaller into 

 males, and both apparently parthenogenetically, i.e. without fer- 

 tilization. In the autumn thick-shelled eggs are produced, which 

 seem to require fertilization, and in the spring develop into 

 females. A large part of the interest which attaches to the Ro- 

 tifera is due to the cilia on the head region. These led them at 

 first to be classified with the ciliated Protozoa, but now they show 

 the relationship which these animals possess to the more highly 

 organized worms, many of which possess larvae strikingly similar 

 in structure to the Rotifera. 



SUBTYPE VI. MOLLUSCOIDEA 



The Molluscoidea (Lat. molluscutn, mollusc, and Gr. eZSo?, 

 form) are sometimes called the Prosopygia (Gr. irpocrco, for- 

 ward, and 7Tvy)} % rum])). Their position in the system of classi- 

 fication is somewhat doubtful, but at present they are pretty 

 generally recognized as belonging to the Vermes, though some 

 scientists consider them a type by themselves. The name was 

 clue to an inadequate knowledge of the structure of these ani- 

 mals, for the shell, consisting of two valves, which is present in 

 some, resembles very superficially the shell of clams, mussels, 

 oysters, etc., belonging to the type Mollusca ; we now know- 

 that the Mollusca and the Molluscoidea are not closely related. 

 The members of this subtype are mostly marine, and are attached 

 so that they cannot move about from place to place. In one 

 class large colonies are produced ; in one, the individuals live 

 singly. The mouth is at the free end of the body, and near it 

 is the anus. In the neighborhood of the mouth is a tentacle- 

 bearing ridge, the lophophore, often horseshoe-shaped, often 

 circular, the most striking external characteristic of the group. 

 Within, there is generally a distinct body-cavitv in which the 

 alimentary canal is suspended. The Molluscoidea may be 

 divided into three classes. 



