OF BRYOZOA. 199 



derance toward the nervous (neural) side (g] which is so largely 

 developed in the one which I have just sketched out. So slight 

 indeed is this obliquity, from front to back, of the tentacular 

 crown of Fredericella, that it seems almost like an accidental 

 one-sidedness ; but an attentive examination of any number of 

 individuals will reveal the un variableness of this character, and 

 always in symmetrical relation to right and left; although not 

 so strikingly prominent in this respect as in Pectinatella. 



Among the asymmetrical forms of Mollusca, and one of the 

 most lowly organized of the series, is the familiar Oyster. 

 Although so comparatively simple in its structure, there are 

 numerous intermediate stages of organization between it and 

 the Bryozoans, some of which, from their peculiar interest, I 

 would be glad to lay before you ; but I must, from want of time 

 and space, content myself with the illustration of a sketch here 

 and there, along the line of upward progress, of such as are most 

 serviceable for our immediate wants. The irregular, ragged out- 

 line, and unsymmetrical shape of the Oyster (fig. 121), would 

 hardly lead one to suspect the unity of relationship to right and 

 left which reigns in its organization. That it has a definite right 

 and left, and that too always corresponding, the first to the Hat, 

 and the latter to the deeper hollowed shell, is easily demonstrated, 

 and without entering into the minuter details of its structure. 

 Having separated the flat valve from the animal, keep it in posi- 

 tion with the right hand, and hold the hollowed valve (sh l ) in 

 the left hand, both so disposed that the thicker part or beak (sh) 

 of the shells is turned from the body and their edges project in a 



menty exterior tube common to the whole colony ; ek, the thinner transparent 

 part of tu immediately about the body of the contained individual ; t, t 1 , f 2 , the 

 slightly oblique single circle of feelers ; c, the calyx, a scalloped membrane which 

 joins the bases of the feelers ; /, the lip at the front of the mouth ; ce, the throat ; 

 st. the stomach ; st*, the bottom of st ; sfi, the anterior end of st ; c/, the valve 

 between st and cZ 1 ; c/ 1 , the cloaca or last division of the intestine ; an, the pos- 

 terior opening of the intestine ; ac, the abdominal cavity ; g, the nervous gan- 

 glion ; r, i-l, the retractor muscles of the head ; r 2 , the retractor muscle of the 

 stomach ; bd, a very young bud ; in, the walls of the body partially doubled upon 

 themselves, invaginated ; d, the posterior end of the individual. Original. 



