APLACOPHORA. 283 



is unequal from the first. The mode of segmentation resembles that 

 of Dentalium and many Lamellibranchs. After 24 hours there 

 appears a median corona of vibratile cilia, while two ciliated areas 

 appear at the cephalic pole and the point of invagination respect- 

 ively. The embryo elongates and becomes divided by two annular 

 constrictions into three segments. The cephalic segment is formed 

 of two rows of ciliated cells ; some of the cilia become longer than 

 the rest, and one finally becomes much larger, and forms the term- 

 inal flagellum. The second segment or velum is formed of a single 

 layer of cells, which have a single row of cilia \ these grow and 

 form the ciliated corona, the chief organ of locomotion. The third 

 or pallial segment is formed of two rows of cells which are entirely 

 covered by fine cilia (pi. 48, fig. 5, larva of 36 hours). In a larva 

 of 100 hours (pi. 48, fig. 6) three imbricated spicules are to be seen 

 on either side of the ventral line, still enclosed in their mother- 

 cells. The spicules increase in number. The conical body elongates 

 rapidly and becomes curved on its ventral surface, while the mantle 

 is gradually reduced, and the embryo falls to the bottom, as the 

 -ciliated corona is unable longer to support it in the fluid. 



Only one of the embryos passed safely through the critical period 

 of metamorphosis, which is on the seventh day. This change 

 consists in the casting off of almost the whole of the external envel- 

 ope of the larva, that is to say, of the cells of the velum and the two 

 rows that form the pallial lobes. Seven dorsal calcareous and 

 slightly imbricated plates were observed in the surviving embryo 

 (pi. 48, fig. 7, plates seen along the right margin). 



Classification. 



The Aplacophora present two divisions of unquestionably higher 

 rank than families. 



Suborder I. Chcetodermatina Simroth (=Ord. Chcetoderma Lank.). 



Spiculose integument continuous all around the body. 



Family Chcetodermatidce, p. 284. 



Suborder II. Neomeniina Simroth (=Ord. Neomenice Lank.). Spic- 

 ulose integument interrupted beneath by a longitudi- 

 nal ventral furrow. Family Neomeniidce, p. 288. 

 In the present condition of knowledge it would seem inadvisable 

 to recognize more than one family in each of the suborders ; but it 

 should be mentioned that Simroth (in Bronn, p. 225) suggests, but 

 does not adopt, a division of the Neomeniidce into four families, thus : 



