VII 



MOLL USCA ONTOGENY 



251 



segmentation is unequal and total, and takes place through the formation of 

 micromeres. The process of gastrulation seems to occur in a manner half way 

 between epibole and invagination. The blastopore marks the posterior end of the 

 larval body, which is divided by two circular furrows into three consecutive regions. 

 The anterior region consists of two circles of cells, and evidently corresponds with 

 the pretrochal area. It is partially ciliated, and carries in the middle a group of 

 longer cilia, one of which is sometimes to be distinguished from the rest as a flagellum. 

 The second region, which consists of a single row of cells, carries a circle of long 

 cilia, and evidently represents the velum. The third region consists of two rows 

 of cells carrying short cilia ; the second row edges the blastopore. At an older 

 stage, the posterior part of the larva appears to be withdrawn into an invagination 

 of the anterior part. The whole or by far the greater part of the body of Dondersia 

 is said to be produced from this posterior part (the "embryonic cone") alone. On 

 this embryonic cone, there appear, first of all, on the two sides of the middle line, 

 three pairs of consecutive imbricated spiculre, still retained in their formative cells. 



Fig. 208. Dondersia toanyulensis. A, Larva 30 hours old. B, Larva 100 hours old. C, Young 

 Dondersia immediately after transformation (7th day), after Pruvot. 



They soon break through these cells, and their number is increased by the appear- 

 ance of new ones anteriorly. The embryonic cone lengthens, becomes curved ven- 

 trally. The anterior part of the body with the velum and the pretrochal area becomes 

 reduced and finally appears as a sort of collar at the anterior end of the body. The 

 larva sinks to the bottom, and throws off the whole anterior part of the body with 

 the velum and the pretrochal area. Such throwing off or resorption of parts of the 

 body which have been of great physiological importance in the larva is very common 

 in the animal kingdom ; see sections on the ontogeny of the Worms (e.g. Nemertina, 

 Phoronis, etc., vol. i. p. 272), of the Arthropoda (Metamorphism of Insects, vol. i. 

 p. 490), and of the Echinodcrmata. 



On the dorsal region of the young Dondersia, seven consecutive, imbricated, but 

 only slightly overlapping, calcareous plates can now be distinguished, consisting 

 of rectangular rods lying close alongside of one another (Fig. 208, 0). This 

 fact is very significant with regard to the shell of the Chiton, which in the adult 

 consists of eight, but in the larva of only seven plates. If it could be proved that 

 the Solcnogastridce pass through a Chiton stage, the view that they are more 

 specialised animals than the Polyplacophora, and are to be derived from Chiton -like 

 forms, would receive almost decisive support. 



Besides the seven dorsal calcareous plates, the young Dondersia has numerous 



