240 HYDROID.E. Part IV. 



naked, being a simple perforation through the outer and inner walls, without any 

 folds or appendages. From the mouth to the base of the body there is one 

 uniform cavity {d) without fold or constriction, or any sign of a distinction between 

 a stomach and a circulatory chamber. When the animal is in full activity and 

 extension, the outer wall {Fig. 10, a) of the body is moderately thick and forms 

 an even layer from the mouth to the base ; but where it forms the outer wall of 

 the tentacle it is very thin, and more lili^e a delicate epidermis, excepting at the 

 tip of these organs where it is very thick, and constitutes the bed of the lasso- 

 cells. The inner wall {Figs. 10 and lO**, h) is almost three times as thick as the 

 outer one, and, like the latter, it forms an even layer from the top to the bottom 

 of the body, the only diversion being its lateral projections into the axis {b^) of 

 the tentacles. When the body is in a swollen and partially retracted state {Fig. 

 lO*") the proportionate thickness of the walls changes considerably, but the fore- 

 going description refers to the normal, and most frequent state. In the tentacles 

 the inner wall occupies nearly their whole bulk, and there, as in the body, is 

 composed of only a single layer of cells. {Figs. lO*" and 10", b^.) We have not 

 been so fortunate as to see the medusoid state of this animal. 



Hislology. — The cells of the outer wall have not been seen, nor does any thing 

 in the wall indicate that it has an organic structure, except at the globular tip 

 of the tentacles, where it is prowded Avith lasso-cells. The cells of the inner wall 

 {Fig. 10*, b) are disposed in a single layer ; they have rounded ends outwardly 

 and inwardly, and vary in breadth according to the degree of contraction of the 

 body. In the tentacles they ai-e also in a single layer {Figs. lO*" and 10°, h^) and 

 have more or less of a truncate conical shape, the end of one serving, as it were, 

 for the base of the next beyond. There are only three or four of these cells 

 in each tentcacle, a peculiarity not to be observed among any of the other 

 Hydroids. 



