M. A. Giard on the Genus Entoniscus. 145 



frequently in the former. Of five infested individuals, four 

 belonged to the male and one to the female sex. But I have 

 learned from my investigations of the Rhizocephala that such 

 statistics, to be of any value, must be founded upon hundreds 

 of individuals collected in the same locality. Now my Grapsi 

 came from various parts of the coast ; and I have examined, at 

 the utmost, two hundred individuals. I believe we may esti- 

 mate at one in thirty the number of crabs which bear the 

 parasite *. 



Sometimes one finds two Entonisci in the same crab ; and 

 in this case one of them has hampered the growth of the 

 other, which is a favourable circumstance for the observation 

 of intermediate stages, always rare among parasitic animals, 

 in consequence of the rapidity of the retrogression. 



Entoniscus Cavolinii, like the species studied by Fritz 

 Midler, is enclosed in a fine membrane in continuity with 

 that which lines the inner surface of the branchial cavity of 

 the crab. It is placed between the liver, the stomach, and 

 the heart of its host. Generally the head is concealed among 

 the hepatic casca, and in part hidden under the anterior ovi- 

 gcrous sac ; the tail is recurved upon the ventral part, and 

 passes under the heart of the crab. The parasite is sometimes 

 on the left, sometimes on the right side of its host, most fre- 

 quently on the left side (three times more frequently) as it 

 seems to me. 



The general form is rather difficult to describe ; moreover 

 it varies according to the age and position of the parasite. 

 We have represented it (PI. X. fig. 1), as accurately as pos- 

 sible, from a living individual twisted upon itself. The 

 colour also varies according to the state of development of the 

 ova with which the animal is almost entirely surrounded ; it 

 is straw-yellow when the ova are but little advanced ; at 

 maturity it acquires the lead-grey tint so well observed by 

 Cavolini. This tint is due to the formation of a peculiar 

 pigment in the embryo. 



We have represented in PI. X. fig. 2 the same individual 

 untwisted in such a manner as to bring the head into its 

 normal position. In this figure the walls of the incubatory 

 cavity and the abdominal plates are supposed to be removed, 



* It is remarkable that none of the Grapsi that I have examined bore 

 a Sacculina. Fritz Miiller remarked a frequent coexistence of Entomsojis 

 porcellance and Lemceodtsais porccllatKB. My zealous collaborateur, J. 

 Prit5, whose attention I had called to this point, has also never met with 

 Sacculina Benedenii. Moreover the Suctoria seem to prefer calm and 

 slightly impure waters; they are much more frequently found upon the 

 various Decapods of the Bay of Penbron, at the Croisic, than on the 

 same crabs collected on the open shore of Pouliguen. 



