THE ACTIXOZOA. 137 



apparent absence of the remains of Hydrophora in the meso- 

 zoic and newer palaeozoic rocks is very remarkable. Some 

 singular organisms, termed Graptolites, which abound in the 

 Silurian rocks, may possibly be Hydrozoa, though they 

 present points of resemblance with the Polyzoa. They are 

 simple or branched stems, sometimes slender, sometimes ex- 

 panded or foliaceous ; occasionally the branches are connected 

 at their origin by a membranous expansion. The stems are 

 tubular, and beset on one or both sides with minute cup- 

 shaped prolongations, like the thecae of a Sertularian. A solid 

 thickening of the skeleton may have the appearance of an 

 independent axis. Allman has suggested that the theciform 

 projections of the Graptolite stem may correspond with the 

 mematophores of Sertularians, and that the branches may 

 have been terminated by hydranths. Appendages which ap- 

 pear to be analogous to the gonophores of the Hydrophora 

 have been described in some Graptolites. 1 



With a very few exceptions (Hydra, Cordylophorci) the 

 Hydrozoa are marine animals; and a considerable number, 

 like the Calycophoridce and JPhysophoridce, are entirely pe- 

 lagic in their habits. 



The Actixozoa. — The essential distinctions between the 

 Actinozoa and the Hydrozoa are two. In the first place, the 

 oral aperture of an Actinozoon leads into a sac, which, with- 

 out prejudice to the question of its exact function, may be 

 termed " gastric," and which is not, like the hydranth of the 

 Hydrozoon, free and projecting, but is sunk within the body. 

 From the walls of the latter it is separated by a cavity, the 

 sides of which are divided by partitions, the mesenteries, 

 which radiate from the wall of the gastric sac to that of the 

 bod} 7 , and divide the somatic cavity into a corresponding num- 

 ber of inter mesenteric chambers. As the gastric sac is open 

 at its inner end, however, its cavity is in free communication 

 with that of the central space which communicates with the 

 intermesenteric chambers ; and the central space, together 

 with the chambers, which are often collectivelv termed the 

 " body cavity " or " perivisceral cavity," are, in reality, one 

 with the digestive cavity, and, as in the Hydrozoa, consti- 

 stute an enter ocoele. Thus an ActinozoQn might be com- 

 pared to a Lacernaria, or still better to a Carduella, in which 

 the outer face of the hydranth is united with the inner face 



1 Hall, " Graptolites of the Quebec Series of North America," 1865. Nichol- 

 son, " Monograph of the British Graptolitidte," 1872. 



