292 Chronicles of Science. [April, 



Free commensals are the most numerous. Little fish live inside 

 jelly fishes, without incommoding their host, or being incommoded ; 

 a whole troop of a particular species may be thus sometimes seen. 

 Similarly a fish called Fierasfer lives inside a Holothurian, and in a 

 large species of Anemone the same observation has been made. 

 Dr. Semper has described, in addition to the little fish inhabiting 

 Holothuriae, several molluscs, which also live in this way; whilst 

 Muller made known Entoconchon, from the Synapta; and Sty- 

 lifer lives on Echinus. The Eemora is a remarkable instance of 

 commensahsm. This strange fish, by means of the sucker on the 

 back of its head, attaches itself to other fish and to whales, some- 

 times to ships, and is thus carried along through rich feeding-grounds. 

 The inhabitants of Mozambique use this fish as a means of captur- 

 ing others, tying a string to it and letting it out into the sea, when 

 it attaches itself to some unsuspecting inhabitant of the ocean, which, 

 together with the Eemora, is speedily dragged to shore. The little 

 crustacean (Pinnotheres), which lives inside the shell of the common 

 edible mussel, has long been known, and various species in this and 

 other countries have excited speculation and fable. There is no 

 doubt a most cordial understanding between the little crab and its 

 host ; and though we cannot go so far as to believe that the crusta- 

 cean acts as a watchman for the Mytilus, warning it when to close 

 its shell, it is yet very evident that there is a close relationship of 

 reciprocal advantage existing between the two. Chsetogaster, the 

 little worm which crawls about on Lymnseus and Planorbis — the 

 common water-snails of our ponds — is a good example of a com- 

 mensal, sticking very close to his friend, feeding on the Cercarise 

 {true parasites) and other matters which accumulate on the snail's 

 body. Many tubicolous Annelids have a commensal, or messmate, 

 who shares their residence ; such are many scale-bearing Annelids 

 — the Polynoina, which ensconce themselves in the tubes of Ghse- 

 topterus insignis, of Terebella nebulosa, and others. One Polynoe 

 was many years since described by Professor Huxley as living on 

 the common Gross-star, and hence named P. astericola. 



The distinction between commensal and parasite is this, that 

 the parasite uses his host for food ; whilst the animals which are men- 

 tioned above, and many others enumerated by Professor Van Bene- 

 den, though often termed parasitic, do not feed upon the tissues 

 of the animal with which they live, and hence have a very different 

 relation to them. Their food consists often of what is rejected by, 

 or is even hurtful to, their hosts ; and though the line between them 

 and true parasites may not be easy to draw sharply, it is yet useful 

 to recognize them under a distinct name, as proposed by Professor 

 Yan Beneden. 



A New Genus of Cervidse. — Mr. E. Swinhoe, who, as consul at 

 Formosa and various stations on the Chinese coast, has done most 



