28 MA BINE AND FISHERIES ■ 



1-2 EDWARD VII., A. 1902 



and female individuals, and that they attained to full reproductive maturity in April. 

 For several months previously the reproductive organs had been gradually developing 

 and ripening their elements, as also for some time afterwards there was a slow decline 

 in the efficiency and size of these organs. While the time he mentions agrees tolerably 

 closely with that of our common clam, it seems somewhat remarkable that the horse 

 mussel should breed late in the season. During the month of September, the sexual 

 characters of Modiola modiolus are very evident. Unlike Mytilus in which the devel- 

 opment in size and colour is chiefly in the dorsal and lateral parts of the mantle, in this 

 species the increase in size is almost entirely confined to the visceral mass. It does not 

 appear possible to distinguish male and female individuals from the closed shell, but 

 when the shell is gaping open one can distinguish them at a glance. The large distended 

 abdomen of the female is a bright orange, while that of the male is yellow. The mantle 

 in each is yellowish, but in the female its edges become more orange, while the gills of 

 both remain brown. I have found no mention of sexual coloration in the clam, but 

 clam diggers have informed me, upon being questioned with regard to this point, that 

 at a certain time in the spring clams are not good to eat, and are greenish in colour. It 

 •will be interesting to discover if this statement has reference to the ripening of the 

 reproductive elements, or if it has reference to another phenomenon that is occasionally 

 produced when clams feed upon a particular species of diatom. 



The sexual elements are ova and sperms (Plate IV., fig. 7). The ova originate in the 

 ovary of the female, and sperms in the testis of the male. Both these organs are situated 

 in the abdomen, round the coils of the intestine. Ripe ova, disconnected and free from 

 pressure, are spherical, but when viewed in number, and more or less subject to pressure 

 from their neighbours or from the cover glass in a microscopic preparation, they are 

 more or less oblong or oval, and measure about -^-^ mm. in diameter (the one in the 

 drawing measured "100 x "120 mm.) The egg is surrounded by a membrane, under 

 which is a pale layer ; then follows yellowish brown granular protoplasm, in which is 

 situated a large pale nucleus containing a nucleolus. The sperm cells are pin-shaped 

 with a large head, and a long filamentous tail. The head is -00.5 mm. long, and is oval 

 in form or top shaped. At the small end there is a smaller constricted part which 

 tapers off to a point, corresponding to that upon which the top spins. In the middle of 

 the larger end of the oval the tail is inserted. This statement is at variance with the 

 observation of Dr. John Wilson in the 4tH Annual Report of the Fishery Board for Scot- 

 land, 1885, where it is stated that the tail originates from the constricted part. Eggs and 

 sperms are shed through special ducts into the sea-water. It is not likely that sperm 

 cells make their way, against the outflow of water, through the exhalent dorsal siphon, 

 or, with the inflow, by way of the ventral siphon, gill slits, &c., to meet the eggs before 

 the latter are extruded. 



May 30, 1901, at Canso, N.S., I fourd sexually mature mussels and clams. I give 

 below a comparison of the measurements I took at the time with those of the horse- 

 mussel given in the text. 



Modiola..^S«'00- -'^O-m. 

 ( sperm "OOo mm. 



Mvtilus l^gg-082 X 090 mm. 

 iviytuus... -j gp^^j^ .QQg3 ^ .QQ27 mm. 



^ j egg 058 X -062 mm. 



^^^^ I sperm "0045 x •0022 mm. 



The measurements of the eggs are those of the 'shortest and longest diameters, and 

 the measurements of the sperm are those of the length and breadth of the head only. 



In all three the tails of the sperm cells are attached to the centre of the big end of 

 the head. In Mytilus the sperm head tapers off to a long sharp point, the outline of 

 the sides being concave rather than straight or convex. In Mya the sperm head tapers 

 to a shorter blunt point, the outline of the sides being distinctly convex. Neither of 

 them possesses the little headed constriction as shown in the sperm head of Modiola. 



Considering the similarity in structure, habits, habitats, &c., there can be little 

 doubt but that the above account, as far as it has been described, might, with tolerable 

 correctness be written also of Mya arenaria. Fertilization, or union of sperm and egg, 

 takes place outside of the animals, in the sea-water. For one egg there are thousands, 



