1884.] NATURAL SCIENCES OP PHILADELPHIA. 191 



acter over the whole surface of the coenajcium, there being no 

 such arrangement of locomotory apparatus upon the lower surface 

 in Cristatella as Prof. Allman describes and figures in the case 

 of C. mucedo.^ In both genera, also, by a delicate manipulation 

 of the light under a high power of the microscope may be 

 detected the fine lines of transverse and longitudinal muscular 

 tissue which form the third and fourth layers of Prof. Hyatt's 

 series, and are visible also under the thinner cell structure of the 

 evaginated polypide. 



As generally accepted, the ectocyst, which, in Pectinatella^ 

 forms a solid and constantly thickening mass of gelatinoid 

 matter, is in this genus thrown off" as a fugitive film, or, more 

 generally, a pavement layer of eflTete matter that supports the 

 colonies and upon which their locomotion is eff'ected. When the 

 young colonies have been liberated from the floating statoblasts 

 in my jars, they float, as has been already described, with their 

 discs at the surface of the water, and this delicate, invisible film 

 spreads upon the surface, uniting the neighboring colonies and 

 forming a common basis of support from which they do not 

 appear voluntarily to remove. In a natural situation on a stream 

 or pond the wind or currents would probably soon waft them 

 against some solid substance which they • would afterwards 

 colonize and inhabit. As has been said, no especial contrivance 

 appears to exist for facilitating the locomotion of these colonies, 

 and, while their power in this respect is, of course, unquestion- 

 able, the writer is inclined to doubt whether it is exercised 

 voluntarily and with a purpose, or is not rather an accidental 

 result of the frequent contractions and expansions of the retractor 

 muscles disturbing the position of alternate portions of the 

 disc. This seems the more plausible, as we do not find in this 

 species any method of prehension in the colonies, but merely a 

 gelatinous or slimy cohesion to the ectocyst. 



At maturity the evagination of the polypide in the species 

 under consideration is complete, leaving not only no " invaginated 

 fold " but exhibiting the whole digestive system of the polyp 



^ " In the middle of the flattened under surface is an oval disc resembling 

 the foot of a gasteropodous moUusk. On this disc, which is contractile 

 and admits of frequent changes of shape, the colony adheres to neighboring 

 objects or creeps about on submerged leaves and stems of aquatic plants, 



etc." 



