3330 



THE MOLLUSKS OR SHELLFISH 



perfect condition have not yet been observed. It is the lowest type of Nudibran- 

 chiata, if indeed it really belongs to that suborder, which is not absolutely certain, 

 some authors even locating it near Natica. Were it not for the form of the larva, 

 one would hardly have supposed that this worm-like creature belonged to the Mol- 

 luska at all. It has neither jaw nor radula, but the mouth is adapted for sucking. 

 The sexes, as in the typical Nudibranchs, are united in the same individual. The 

 female element (b] is somewhat anterior, the male organ (c) being at the opposite 

 extremity. The central portion of the animal serves for a certain time as a nursing 

 pouch for the embryos, which subsequently are extruded at an opening at the pos- 



JS 



Synapta digitata, WITH PARASITIC Entoconcha AND 



MIDDLE PORTION ENLARGED. 



terior end. Only one out of every 

 hundred or two hundred speci- 

 mens of Synapta is infested with 

 the Entoconcha. Pelseneer locates 

 it next to Eulima among the 

 Pectinibranchs. 



COVERED-GII/L SUBGROUP 

 SUBORDER Tectibranchiata 



This constitutes the second 

 suborder of Opisthobranchiata, 

 and includes certain mollusks- 

 characterized by the possession of 

 a lateral gill, protected by the 

 mantle or a shell. The members 

 of the group are hermaphrodite 

 animals, and differ from the 

 Nudibranchs, not only in their 

 respiratory organs, but also in 

 the possession of a true mantle, 

 which, in the Nudibranchs, is 

 represented by the dorsal shield. 

 On the other hand, they show 

 affinity with certain Nudibranchs 

 in regard to the position of the 

 rhinophores, the lateral expan- 

 sions of the body, and the radula. 

 Nearly all the Tectibranchs pos- 

 sess a shell which sometimes is 

 wholly concealed beneath the 

 mantle, sometimes partly covered, 

 and, in many instances, wholly 

 external. Some are carnivorous, 

 others entirely phytophagous or 

 vegetable feeders; they mostly 



