744 NATURAL HISTORY OF AQUATIC ANIMALS. 



dance; the months of May, June, and July may, however, be regarded as their principal spawning 

 months. Ripe spawn may be sparingly obtained in the latter part of August, and even np to 

 the first of October, but the three months mentioned are the periods during which the experi- 

 mentalist ought to be in the field prepared for work in this, the latitude of Washington. What 

 amount of variation from this period may be made manifest as we go north or south along the 

 eastern coast of the United States I am unable to state; and what amount of local variation may 

 also be due to causes of a purely local character I am also unable to say, not having examined 

 the Oysters at a sufficient number of localities to make such facts as I may possess of any 

 value. 



218. THE OYSTER-CRAB AS A MESSMATE AND PURVEYOR. 



It is many years since Mr. Say named the little Oyster-crab Pinnotheres ostreum, and its habits 

 since that time seem to have excited but little interest. Professor Verrill, in his observations pub- 

 lished in the "Report of the United States Fish Commissioner for 1871-'72," records the fact that 

 it is the female which lives in the Oyster, and that the male, which is smaller and unlike the 

 female, especially in the form of the abdominal segments of the body, is rarely if ever seen to 

 occur as a messmate of the Oyster, but that he has seen it swimming at the surface of the water 

 in the middle of Vineyard Sound. He also says that they occur wherever Oysters are found. 

 This singular little crab has quite a number of allies which inhabit various living mollusks, holo- 

 thurians, etc., of which admirable accounts are given by Van Beneden in his work on "Animal 

 Parasites and Messmates," and also by Semper in his treatise entitled "Animal Life." 



QUADRUPLE COMMENSALISM. The Oyster-crab is a true messmate, and it is in the highest 

 degree probable that the presence of these animals in the mantle cavity of the Oyster is to be 

 regarded as advantageous rather than otherwise. The animal usually lives between the ventral 

 lobes of the mantle of its host, into which the four lobes of the gills and palps also depend, 

 and, as will be seen from the following observations, may be the means of indirectly supplying 

 its passive protector with a portion of food. During a trip down the Chesapeake in July, 1880, 

 while I was with the Fish Commission vessel, some Oysters were dredged up by the crew which 

 contained some Oyster-crabs. In the case I am about to describe the included crab was a female 

 with the curiously expanded, bowl-like abdomen folded forward under the thorax, partially 

 covering a huge mass of brownish eggs. Upon examining these eggs, what was my astonish- 

 ment to find that they afforded attachment to a great number of compound colonies of the 

 singular bell animalcule, Zoiithamnium arbusculum. Upon further examination it was found that 

 the legs and back of the animal also afforded points of attachment for similar colonies, and 

 that here and there, where some of the individuals of a colony of Zoiithamnium had been sepa- 

 rated from their stalks, numerous rod-like ribriones had affixed themselves by one end. In this 

 way it happens that there is a quadruple commensalisin established, since we have the vibriones 

 fixed and probably nourished from the stalks of the bell animalcule, while the latter is benefited 

 by the stream of water drawn in by the cilia of the Oyster, and the last feeds itself and its protege, 

 the crab, from the same food-laden current. Possibly the crab inside the shell of its host catches 

 and swallows food which in its entire state could not be taken by the Oyster, but in any event 

 the small crumbs which would fall from the mouth and claws of the crab would be carried to 

 the mouth of the Oyster, so that nothing would be wasted. 



We must consider the crab with its forest of bell animalcules in still another light. Since 

 the animalcules are well fed in their strange position, it is but natural to suppose that they would 



