432 CRUSTACEA ENTOMOSTRACA. 



gives way before it. Through the aperture so produced the body of the 

 parasite projects into the outer world (Figs. 277 and 278, B). 



The Sacculina now enters on its final phase of existence in which it is 

 known as the external Sacculina. It forms a flattened oval mass about 

 the size of the terminal joint of the little finger, whose long axis is trans- 

 verse to that of the abdomen. It is connected with its host by a short 

 pedicle which passes from one end of the shorter axis through the abdom- 

 inal wall, and is continued into the system of roots (cf. Fig. 278, B). At 

 the opposite end of the short axis from the pedicle is the cloaca, which, 

 until a brood of young has been produced, is closed by a chitinous plate 

 projecting at the sides, beyond the lips of the cloaca. This leads into the 

 brood-chamber (Fig. 278, B, b) surrounding a central visceral mass which 

 projects from the region of the pedicle into it. -The wall bounding the 

 brood-chamber externally is known as the mantle. It is connected with 

 the visceral mass by a mesentery which is of small breadth but ex- 

 tends from the region of the pedicle nearly to the cloaca along the side 

 which is turned towards the right side of the crab. The Sacculina thus 

 lies in a definite relation to its host. In the visceral mass lie the large 

 paired ovaries, the ducts of which are connected with multilobed digi- 

 tate cement glands (dr) and open on either flattened face (Fig. 278 B). 

 Near its base lie two cylindrical testes. The vasa def erentia open into the 

 brood chamber. A ganglion is situate on one side of the plane of the 

 mesentery in the visceral mass and supplies nerves to it and to the mantle, 

 which they reach through the mesentery. 



According to Delage's view all but the first batch of eggs are fertilized 

 by the spermatozoa of the animal which produces them, and this is effected 

 before the ova leave the oviducts ; the spermatozoa finding their way 

 into the latter from the brood-chamber. When a batch of eggs is ripe 

 the cuticular lining of each of the cement glands is shed all in one piece, 

 and the multilobed digitate bag so produced becomes distended with the 

 eggs in its passage to the brood-chamber. The two batches of eggs each 

 contained in its cuticular sack, lie in the brood-chamber on either face of 

 the flattened visceral mass. They are held in position by minute hooked 

 prominences (retinacula) which project from the inner lining of the brood- 

 chamber, and are supplied with oxygen by the regular contraction of the 

 mantle. From the eggs emerge the nauplius larvae above described. 



It appears that no very long time elapses between the entry of the 

 parasite into the crab and its taking up its position under the gut. Accord- 

 ing to Delage the Sacculina becomes external at the age of 20-22 months, 

 the host being about four months older. The first brood is pro- 

 duced four months later, and other broods succeed, during the summer 

 at intervals of 4 or 5 weeks. A Sacculina becomes external and begins to 

 produce broods in the late summer, and the production of broods is con- 

 tinued during the next summer. At the end of this second summer, being 

 aged rather more than three years, it dies. While the Sacculina is ex- 

 ternal and producing its broods of nauplii the drain on the resources of 

 the host is greatest, and the Crab does not increase in size or moult 

 though it is not necessarily prevented from producing its own young.* 



* In the case of the crab Inachus which is infested by a species of 

 Sacculina, permanent infertility results from the action of the parasite. 

 Cf. p. 445. 



