216 Mr. W. S. Kent on the Relationship of 



Actinaria : such a differentiation, however, is, to say the least of 

 it, carried out to a considerably less degree. There are certain 

 sponges which are invested with a pellicular and somewhat 

 tough dermal membrane; but in the majority of instances and 

 in the most highly organized representatives of the class, such 

 an amount of differentiation is by no means recognizable. The 

 new and very beautiful siliceous sponge Holtenia Carpenteri^ 

 recently dredged by Prof. Wyville Thomson and Dr. Car- 

 penter in the Shetland seas, is a good example of this type of 

 organization. A fine specimen of this highly interesting form, 

 immersed in spirit, has recently been consigned to the National 

 Collection ; but the appearance of the body -substance to the 

 unassisted eye is that of a simple homogeneous mass of sarcode, 

 showing a tendency to fracture in every direction, aggregated 

 upon the dense network of sj^icula which support it*, some- 

 thing entirely different from the ajipearance of an Actinarian 

 viewed under similar conditions. This form, moreover, possess- 

 ing a single very large flue or osculum, would be regarded 

 by Ilackel as correlative with a solitary Actinia ; and the 

 large size of this species (the body of the sponge proper mea- 

 suring some four inches in both length and diameter, and 

 having a general cxcurrent aperture of the width of an inch 

 and a half) would be admirably adapted for comparison with 

 some \\\\.^Q A. crassicomis \ the differences existing between 

 two such similar structures, however, as in the examples of 

 Spongilla and an ordinary Actinia^ have been already so 

 clearly indicated as to render further comparison unneces- 

 sary, except, perhaps, that, in the living condition, the firm 

 elastic ectoderm of crassicornis would offer a most striking 

 and distinctive feature by the side of the low-organized and 

 glaiiy sarcodic investiture of Holtenia. 



While on the subject of the dermal investitm'e of the 

 sponges, it will not be out of place to remark that in those 

 instances where the demial membrane attains a comparatively 

 high degree of development, it has been observed, most gene- 

 rally, to possess a peculiarity essentially its own, and one not 

 met with in any Coclenterate organism. This is its property 

 of being able to separate its individual component particles at 

 any point whatever, and so form the pores,/, figs. 1 & 2, for 



* It has been suggested to mo that the spicular skeletal system of the 

 Spongiadas seems to indicate their close relationship to the Actinozoa. 

 Siliceous spicula most closely resembling those of the Spongiadte, however, 

 are of common occurrence in that section of the true Protozoa known 

 as the Radiolaria, the great spicule-secretiug division of the Actinozoa 

 (the Alcyouaria), on the other hand, never being found to possess any 

 thing like an approach to such forms. 



