IX MOLLUSCA 325 



and at the same time a degenerate group, in some of which the 

 ventral ciliated groove is retained throughout life; but they have 

 been shown in one case to possess a typical Trochophore larva. This 

 case is that of Bonder sia, and the Trochophore is gradually converted 

 into the adult form by the elongation of the post-trochal region. 



SCAPHOPODA (Dentalium) 



- The Scaphopoda, with tubular shell and mantle, are represented 

 by Dentalimn and a few closely allied genera. The development of 

 Bentalium has been worked out by Wilson (1904). It is practically 

 of world-wide distribution, being found in muddy bottoms. Wilson 

 found the eggs of the Mediterranean species ripe in June, and he 

 gives the following description of them. They are yolky and deeply 

 coloured by pigment which varies in tint from olive-green to 

 brownish-red. When dehisced from the ovary the egg is almost as 

 flattened as a biscuit, though one side is more flattened than the 

 other, and this side is proved subsequently to be the vegetative pole. 

 In the centre of each flattened surface is a white non-pigmented 

 area. 



After remaining in sea- water for from twenty to thirty minutes the 

 egg becomes spherical and bursts its ovarian membrane or chorion. 

 A jelly-Kke layer which surrounds the egg then swells up, and the 

 egg now looks like a sphere with white poles and a broad ring of 

 pigment. But when the egg is fixed in picro-acetic and cut into 

 sections, the two poles are seen to be widely different. At the 

 vegetative pole there is a dense mass of cytoplasm, devoid of yolk, 

 which is continuous with a thin layer of clear cytoplasm surround- 

 ing the egg. This mass of cytoplasm also extends upwards through 

 the egg to the germinal vesicle or nucleus, which is situated near 

 the animal pole and surrounds it. At the animal pole there is a 

 minute disc of cytoplasm free from yolk, which is far too small to 

 account for the large white area seen in the living egg in this region. 

 This latter must owe its appearance therefore to the presence of yolk 

 granules of white colour. 



As the egg lies in sea- water the cytoplasm of the animal pole 

 slowly increases in amount, seemingly by a radial inflow from sur- 

 rounding regions. The wall of the nucleus now breaks down and 

 the first polar spindle is formed. Things are now at a standstill 

 nntil the egg is fertilized, when the two polar bodies are formed one 

 after the other. The spermatozoon enters at the vegetative pole. 

 From this pole a pillar of granular cytoplasm extends upwards and 

 becomes temporarily confluent with the cytoplasmic area at the 

 upper pole. This pillar is in large measure produced by the material 

 which was contained in the nucleus of the unripe egg, and which was 

 extruded when the nuclear wall broke down. 



The first cleavage occurs half an hour after fertilization, and is 

 vertical. At the same time the lower white pole of the egg is cut off 



