the Sponges to the Corals. 207 



an ideal sponge-body {Spongilla), bearing a single osculum, 

 and of an ordinary Actinia will present to the reader such 

 analogies or homologies of form and structure as may appear 

 to exist, and will ■ aid materially in the institution of com- 

 parisons. 



These two forms may be described as being so far analo- 

 gous that, in longitudinal section, they both present the same 

 conical outline, that the summit of each cone is provided 

 with an aperture, a, and that in both instances this aperture 

 communicates with an inferior cavity, J, which leads again 

 into ulterior ramifications marked c, d ; beyond this, however, 

 analogy fails to assist us ; and an inquiry into the functional 

 properties of these regions demonstrates them to be the very 

 opposite to homologous. 



The researches of Huxley, Grant, Carter, Bowerbank, and 

 other naturalists have long since demonstrated the essential 

 characters of a sponge-body to be the following : — 



In addition to the large apertures, or aperture, as at fig. 1 a, 

 the dermal membrane,^, is perforated by an indefinite number 

 of smaller ones (pores), marked y"; these communicate (by a 

 series of canals, of various forms and dimensions, fig. 1, e, c^, c, 

 which traverse the deeper layers of the sjjonge-body) with the 

 osculum, a, by means of the central excurrent cavity, h. A 

 flow of water, when the sponge-mass is in a healthy condition, 

 is constantly setting in at the pores, drawn by the ciliary 

 action progressing in the chambers marked r/, and, having 

 traversed the interstitial canals and cavities in the body- 

 mass, debouches into the central excurrent cavity J, and 

 is finally expelled from the organism at the osculum or ex- 

 current aperture, a*. 



These currents, according to the observations of Dr. Bower- 

 bank, are exercised in two different modes — the one being 

 vigorous and of comparatively short duration, when the animal 

 is feeding, and the other very gentle and persistent, and being 

 evidently subservient to purposes of respiration only ; and the 

 last-named author, whose extensive experience with this class 

 of animals is so eminently conspicuous in his excellent and 

 exhaustive '■ Monograph of the British Spongiad^,' states that 

 in no single species which he has had the opportunity of ex- 

 amining in a fresh and vigorous condition has he failed to 

 detect these currents. 



The same writer also ably proves that the imbibition of the 

 surrounding fluid during the energetic action of the sponge is 



* Separate ciliated chambers cannot be said to be essential to the fully 

 developed sponge-organism, the ciliated cells in many forms being equally 

 distributed throughout the interstitial canal-system. 



