INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 57 



and an anterior tuft of cilia ; a remnant of the segmentation cavity 

 is still to be observed at the superior end. 



As the embryo elongates, the mouth (blastopore) passes, according 

 to the author, on to the ventral side ; losing its rounded form it 

 becomes oval, and then finally cleft-like, and comes to lie in a kind of 

 deep groove. Being set close to the ciliated ring, it may be observed to 

 have the form of an elongated tube, opening by its interior orifice into 

 the digestive tract, and by its exterior to the surrounding medium. 

 The walls of this tube soon fuse, and the cells form a plate which lies 

 between the body- wall and the wall of the enteric tract. The mesoderm 

 has meanwhile commenced to develop ; arising from the lateral and 

 lower cells of the endoderm, without, however, yet forming a con- 

 tinuous layer. 



The next step is that in which the embryo elongates, its outer 

 cells diminish in size, and the cilia set up an active rotatory move- 

 ment. A circular groove soon separates the anterior from the posterior 

 portion of the body, and the whole of the ventral side forming a 

 flattened plate becomes developed into the foot of the larva ; at the 

 same time there appear on the dorsal side six or seven transverse 

 grooves, which divide the back of the larva into a corresponding 

 number of half-rings. At this time the embryo breaks through its 

 chorion. The enteric tract arises from the gastrula invagination ; the 

 muscles become developed in the mesoderm, and the nervous system, 

 which consists, even at this early period, of two lateral and of two 

 median trunks, together with cephalic ganglia, apj^ears to arise chiefly 

 from the large-celled plate which is developed at the point where the 

 primitive mouth disappears. Even before the formation of the foot 

 two black dots may be seen, which are evidently eyes and are pro- 

 vided with small lenses. 



In this condition the larva may swim about for an indefinite time ; 

 when it fixes itself by its foot the ring of cilia disajipears, and the 

 spicules which form the shell begin to be developed. Appearing 

 during the larval stage, in some forms, the calcareous spicules have at 

 first the form of simple spicules, first developed in the cephalic region. 

 "When enough are present to form a closely set group, they fix their 

 lower ends deep into the tissue. After these bodies are developed 

 there appear a median and two lateral minute calcareous plates in 

 each division of the body, which soon fuse and form the proper shells, 

 while the earlier spicules arc forced to the margin of the body, partly 

 to disappear. 



It is finally stated in this preliminary communication that the 

 whole of the dorsal surface of the embryo is covered with largo 

 cylindrical cells, altogether similar to the cells found in the mantles 

 of other molluscan embryos. 



Spermatogenesis in Paludina vivipara.* — In a memoir referring 

 to what has been previously written on tliis subject, and summing up 

 his own results, M. Duval says that his previous conclusion — that 

 the two forms of speiTaatozoa noticed in this mollusc are but stages 

 in the development of a single form — has been since refuted. 

 • 'Rev. Sci. Nat.,' i. (1870) p. 211. 



