vii MOLLUSC A ONTOGENY 261 



now added. The region between the two ciliated bands also carries cilia and forms 

 an adoral ciliated zone. 



A further stage of development is depicted in Fig. 217. The rudiment of the 

 pedal ganglion can be recognised as an ectodermal thickening on the ventral side, 

 and that of the gill as a thick epithelial ridge. The stomach has formed a caecum 

 posteriorly, and the narrow mid-gut has formed a loop. The two auditory vesicles 

 containing otoliths have arisen between the mouth and anus as ingrowths of the 

 ectoderm which have become detached. The mesoderm consists of branched muscle 

 cells, branched cells of connective tissue, the primitive kidney and the still 

 undifferentiated cells of the mesoderm streaks. 



The ectodermal thickening, which represents the rudiment of the pedal ganglion 

 at a later stage, becomes rounded off and detached from the ectoderm, at the same 

 time becoming surrounded by the cells of the mesoderm streak, which have rapidly 

 multiplied, and which unite in front of it to form a median mass of cells. This 

 median mass of mesoderm cells increases greatly by rapid division, bulging forward 

 the ectoderm in the anterior ventral region, and thus forming the rudiment of the 

 foot. In the growing branchial fold slits occur, a single slit appearing first, and 

 another soon following in front of the first. The further development of this 

 larva is unknown. 



The development of other marine bivalves runs very much the same course as 

 that of Teredo, the same larva being formed. The ciliated band is very strongly 

 developed in all marine bivalves (Teredo, Ostrea, Modiolaria, Cardium, Montacuta, 

 etc.), and is generally carried by a collar-like expansion of the integument, or velum, 

 which is often divided into two lateral lobes. The velum, which on account of the 

 band of strong cilia it carries is the locomotory organ of the free-swimming larvae of 

 these Lamellibranchs, can be protruded out of and withdrawn into the shell. 



Among fresh-water Lamellibranchs there is one form, Dreissensia polymorpha, 

 whose larva is free-swimming and carries a well-developed velum. This form is 

 said to have migrated from salt to fresh water in (geologically speaking) recent 

 times. 



Special arrangements are found among the other fresh-water forms. The eggs of 

 Pisidium and Cyclas, for instance, develop in special brood capsules in the gills of 

 the mother animal, and leave these as young bivalves. The Trochophora stage is, 

 nevertheless, passed through, but the velum, not being used for locomotion, remains 

 rudimentary. 



2. Ontogeny of Cyclas cornea (Figs. 218 and 219). We shall here only mention 

 the points in which the development of Cyclas differs from that of Teredo, and 

 describe such observations as complete those made on the latter. The blastula 

 consists of a cap of small cells (ectoderm cells) and a floor made of three large cells, 

 one very large primitive endoderm cell and two symmetrical primitive mesoderm 

 cells. The primitive endoderm cell yields through fission a disc of endodermal cells. 

 The two primitive mesoderm cells become overgrown by the ectoderm cells, and 

 thus reach the segmentation cavity. The endoderm invaginates in such a way that 

 a slit-like blastopore arises, which reaches from the region of the future mouth to 

 that of the future anus. This blastopore closes completely. The oesophagus arises 

 as an ingrowth of the ectoderm. A Molluscan Trochophora is formed with a 

 shell gland, a rudimentary foot, a mid-gut, a stomach, anus, primitive kidney, and a 

 neural plate. The velum is reduced to a ciliated area lying at the sides of the 

 mouth (Fig. 218) ; this reduction is connected with the fact that the Trocho- 

 phora of Cyclas is not a free-swimming larva, for the eggs of Cyclas pass through 

 the whole course of their development within the gills of the mother animal. 

 Above the neural plate the ectoderm cells are large and flat, and form a projecting 

 cephalic vesicle. The mesoderm consists of (1) scattered cells, which lie under 



