22 ANIMAL PAEASITES AND MESSMATES. 



has made us acquainted with another messmate of the 

 medusae which greatly resembles an Idothea. 



Another kind of commensalism is that of the 

 Dromiae. These crabs are of the ordinary size, and 

 lodge, from their earliest youth, under a growing family 

 of polyps, which increases with them. This colony has 

 for its principal foundation a living Alcyonium, which 

 covers the carapace, and as it develops, adapts itself 

 perfectly to all the inequalities of the cephalothorax ; 

 one might consider it an integral part of the crab. 

 Sertularise, Corynes, Algae, develop) themselves on this 

 Alcyonium, and the Dromia, masked by this living rock 

 which it carries on its shoulders like the fabled Atlas, 

 marches gravely in pursuit of her prey. She has no 

 fear of arousing the attention of her enemies. The 

 greatest vigilance cannot prevent the sudden attack of 

 these dangerous neighbours. There is in the Mediterra- 

 nean a species which sometimes comes to our coast. 

 They are also known in the Indian Seas and in the 

 Northern Pacific. Eumphius named the dromia Cancer 

 lanosus ; it is, said he, a crab which carries grass or 

 moss on its back. It is also mentioned by Eenard. 

 Dana has observed a sea-anemone covering a crab in the 

 same manner as the Alcyonium does the dromia, and 

 which is not less dangerous. The mode of life of this 

 anemone has procured for it the name of Cancrisocia 

 expansa. In the north of California, a crab (Cryptoli- 

 thoides typicus) covers itself in the same manner with a 

 living cloak which hides it from view, and under cover 

 of which it surprises those whom it attacks. It has 

 already cleared the ground of its prey before any alarm 

 has been given to the neighbourhood. 



