ISOPODS OF NORTH AMERICA. 579 



Parasitic in the incubatory pouch of Synidotea nodulosa (Kr0yer). 



When the Idotea infested with the parasite is laid on its dorsal side 

 and presents the ventral side to the observer, one separates the live 

 pairs of incubatory lamella' and perceives, on a level with the second, 

 third, and fourth thoracic segments, a ])ody re.o'ularh^ oval, slightl}'' 

 attenuated at the two extremities of the long axis, and presenting- an 

 appearance very indistinctly segmented; it seems completely filled 

 with a compact mass of little grains, yellow in alcohol, which are the 

 eggs distending the incubatory cavity. Toward the middle of the sur- 

 face, which is not applied to the ventral part of the host, and a little 

 toward the base, one notices the mark of a dead white. If one detaches 

 this oval body and examines the other side, one sees that it is divided 

 longitudinalh" by a groove, which extends from one extremity to the 

 other. On several individuals one finds, generally toward the extrem- 

 ity the nearest to the head of the host, a very small Isopod, scarcely 

 visible to the naked eye, measuring 0.85 mm., but presenting all the 

 typical characters of the group. This last is the male Clypcouiscus!^ 

 while the oval mass, tilling the incubatory cavity of the host, is the 

 female, reduced to a simple incubatory sac tilled with embryos. 



When the female becomes adult it takes the form figured, repre- 

 senting the dorsal and ventral sides. It is then a globular mass, 

 which appears divided longitudinally by a groove, scared}^ visible on 

 the dorsal side and interrupted toward the cephalic region l)y a white 

 mass, the dorsal organ. This little mass, somewhat thick, the histolog- 

 ical composition of which we have not determined because of the state 

 of preservation of the specimen, is all that remains of the visceral part 

 of the female, the ovary, digestive tube, etc. Outside of this there is 

 nothing more than the body wall transformed into an incubatory 

 envelope, preserving nothing to recall the primitive Isopod, neither 

 segmentation nor appendages. However, if one examines the female 

 on the ventral side, considering its position in relation to the host, we 

 see that the dorsal groove continues on this side and assumes an 

 appearance altogether special. Immersed between the lateral masses 

 tilled with embryos, it is formed by the superposition of a system of 

 small lamella^ which one can easilv enough separate, without tearing, 

 and which permits of eniptying the female completel}' of eggs; it is 

 the opening of the incubator}" cavity. 



We have represented this system of lamelhe as we have been able 

 to expose it under the microscope, after having removed the dorsal 

 surface of the female. This long opening terminates plainly in two 

 exti'emities between the anterior and the posterior bosses of the 

 animal, and the way in which the lamella^ which form it lie over each 

 other in regular order is rather complicated. Very small at the two 

 extremities of the opening these lamelhv become larger toward the 

 ventral part; they are ten or eleven pairs in number. 



