250 COMPARATIVE ANATOMY CHAP. 



nuclei ; these belong to a double row of cells on which is developed the preoral 

 ciliated ring which, in Molluscs, is called the velum (Fig. 207 A). 



(6) At a later stage, the blastopore appears shifted somewhat towards the ventral 

 side, and an inward growth of ectodermal cells begins at its edge ; this is the 

 commencement of the formation of the ectodermal stomodseum. At the posterior 

 and upper edge of the blastopore, there is, in the figure, a cell lying between the 

 endoderm and the ectoderm ; this is, no doubt, a mesodermal cell (B). 



(c) The larva elongates ; a distinct stomodseum (embryonic oesophagus), leading 

 through the blastopore into the archenteron, is formed by the continuous growth 

 inward of the ectodermal cells ; this organ becomes shifted still further forward 

 along the ventral surface (C). 



(d) Fig. 207 G is an oblique section from an anterior upper to a posterior lower 

 point through a slightly older larva, which shows the stomodseum, and, at the sides 

 of the blastopore, the first mesoderm cells. These are probably derived from the 

 endoderm, and are symmetrically placed at the two sides of the blastopore. 



(e) A median section through the next stage (D) shows no mesoderm cells as yet 

 in the median' plane. The mouth, however, appears shifted forward along the 

 ventral side as far as the ciliated ring or velum, the double row of cells in the latter 

 being very clear. 



(/) Transverse section of an older stage (H). The mesoderm cells have increased 

 in number, and are arranged in two groups at the sides of the stomodseum, between 

 the ectoderm and the endoderm. 



(g] At a later stage, a longitudinal section of which is given in Fig. 207 E, the 

 principal feature is a stronger development of the mesoderm, in which a space, the 

 body cavity, now appears. A bulging backward of the stomodeeum forms the first 

 rudiment of the radular sac. Behind the mouth, a sac-like depression is formed, 

 evidently by the ectoderm ; this has been called the pedal gland, although it has 

 not yet been discovered what becomes of it in the adult animal. 



(h) When the body cavity forms, the cells of the mesoderm become divided into 

 two layers, the inner visceral layer becoming applied to the intestine, and the outer 

 parietal layer to the ectoderm (cf. Fig. 207 I). In the transverse section, we see, 

 deep down in the ectoderm, the first rudiments of the pleurovisceral cords. The 

 pedal cords arise in the same way, and anteriorly, in the cephalic area, which is 

 encircled by the preoral ciliated ring, the rudiments of the supra-cesophageal 

 central nervous system form as a neural plate, i.e. as a thickening of the ectoderm, 

 which carries a tuft of long cilia. 



(i) At later stages (F, K, L), the central nervous system with the pleurovisceral 

 and pedal cords become detached from the ectoderm and take up their mesodermal 

 position. The rudiments of seven shell-plates appear on the back as cuticular 

 formations ; the eighth only appears later. A posterior invagination of the ectoderm 

 represents the rudiment of the proctodeum (the embryonic hind-gut with the anus). 

 The first teeth appear in the radular sac. The whole of the cephalic area and the 

 region of the foot become covered with cilia. On the dorsal ectoderm, on the parts 

 that are not covered by the shell-plates, the first calcareous spines appear. In the 

 posterior part of the body, a great accumulation of mesodermal elements evidently 

 marks the position of a formative mesodermal zone. 



At this stage, the larva leaves the egg envelope, and swims about freely, and, on 

 the degeneration of the ciliated ring, sinks to the bottom transformed into a young 

 Chiton. During this last transformation two lateral larval eyes appear on the 

 anterior ventral side of the body. The development of the circulatory system, the 

 nephridia, the genital organs and the ctenidia has not been followed. 



2. Solenogastres. The ontogeny of this order is as yet only known through 

 a very incomplete account of the development of Dondersia banyulensis. The 



