3. The Case of Eriphia spimfrons. H(\ 



But now to return to the question of the association of Entoniscus and Sacculina on 

 the same host. We see from the table given above, that the fact of this association is com- 

 pletely brought out by Dr. Duncker's figures, since out of 47 crabs infected with Entoniscus 

 16 had also Sacculina, that is about one third, while out of 2554 males and females which 

 were presumably without Entoniscus only 47 carried Sacculina, that is about one fifty-fourth. 



I regard therefore the results of these statistics as a complete vindication of the fact 

 that there is a marked tendency for the association of the two parasites upon the same 

 individual host. 



The explanation of this curious association given by Giard & Bonnier is of great in- 

 genuity. These authors hold that the Entoniscids are derived from a group of the Crypto- 

 niscinae like Liriopsidae (see Chapter 6 of this Monograph), which are parasites of the Rhizo- 

 cephala. The Liriopsidae, as is already known and as I have continually observed, may be 

 fixed either on the Rhizocephala themselves or else on the body of the infected crabs near 

 the attachment of the Rhizocephala; in the latter case they nourish themselves on the internal 

 roots instead of on the external body of the Rhizocephala. Now it is supposed that the 

 Entoniscids in the course of their descent from the Liriopsidae have gradually forsaken the 

 Rhizocephala as hosts for the crab itself, so as to finally become parasites of the Brachyura 

 and not of the Rhizocephala. But they still retain a trace of their old habits in showing a 

 selective preference for crabs infested by Rhizocephala. Whether this explanation is true or 

 not (and it is hard to find a prettier), the fact is certain that Rhizocephala and Entoniscids 

 are so frequently found in association on the same individual host that their simultaneous 

 occurrence must have some definite cause other than chance; nor does it seem that they mu- 

 tually benefit one another in any way, as they are not dependent on one another for nutri- 

 ment and the presence of two such large parasites on one host would more likely be of 

 disadvantage to both in this respect. At any rate they are certainly not necessary to one 

 another either as associates or enemies, as is proved by their generally occurring on separate 

 hosts, and the fact of their simultaneous occurrence, merely in so great a proportion that a 

 purely chance coincidence is ruled out, points emphatically to a rudimentary condition such 

 as Giard suggests, rather than to a vital organic connection between the two parasites. 



I am however inclined to put forward an alternative explanation which does not involve 

 any phylogenetic theory. It is a well-known fact that the presence of Sacculina in a crab 

 retards the growth and consequently diminishes the number of moults of the host. In conse- 

 quence of this, crabs which harbour a Sacculina are generally covered with various kinds of 

 animal and vegetable growths. 



Now the Entoniscid parasites of the crabs enter by way of the gills, and doubtless in 

 the early stages of development before they have penetrated into the thoracic cavity of the 

 crab, they would be liable to removal or injury by the constant moulting of the host. But 

 if an Entoniscid were to enter a crab containing a Sacculina interna which had already begun 

 to inhibit the growth of the crab, its liability to removal and injury through the moulting of 



