280 NOTES AND MEMORANDA. 



Alcyonaria, and begins with a brief general description of the skeleton 

 of Actinozoa. This may occur in either of the following positions : — 



o w-.,^-*" ^!.^^ ^'1"f ^ '"'^''''^ ''^ ''^^ entoderm! Entoskeleton. 



2. Within the entoderm j 



3. Between the entoderm and ectoderm — 1 



a Secreted from entoderm cells only . . Mesoskeleton. 



0. From both layers j 



c. From ectoderm cells only J 



4. Within the ectoderm "I jrcfoajjeieton 



5. On the free (outer) surface of the ectoderm / 



Of these only the third and fifth kinds occur in Alcyonaria. 



The mesoskeleton in the simple forms consists of " a thinner or 

 thicker layer between the ectoderm and entoderm, which, after removal 

 of all the cellular elements of the body, retains the form of the polype, 

 since it extends between all the folds of the two primary cell-layers." 



In those compound forms which have the polypes connected by 

 stolons, the mesoskeleton exists in tbe form of a thin lamella between 

 the two layers of the stolons. In the species in which the polj'^pes are 

 united into a broad plate-like colony by means of a solid mass or 

 coenenchyma, the greater part of the latter is formed by the meso- 

 skeleton, which is covered externally by ectoderm, while within are 

 contained the nutritive canals, lined with entoderm, by means of 

 which the polypes are placed in communication with one another. 



In a few forms, the mesoskeleton consists merely of a hyaline or 

 fibrillar substance [Monoxenia, Cornularia). In other cases calcifications 

 or spicules are developed, the arrangement of which differs greatly, 

 both in various portions of the same polype or of the same zoantho- 

 deme, and in the various genera and species. The spicules are often 

 scattered singly in the hyaline matrix, but often, on the other hand, 

 exist in such great numbers as to give the whole skeleton a firm, cork- 

 like consistency, its form being then but slightly altered by drying. 

 In many cases the separate spicules undergo fusion, and form a firm 

 continuous framework, which may replace, to a greater or less extent, 

 the hyaline matrix (^Tuhijjora, Pseudaxonia). In a few forms there 

 is a continuous mesoskeleton, not due to the fusion of originally 

 distinct calcifications. The free part of the polypes and tbe pei'ij)heral 

 portions of the zoanthodeme never undergo conversion into horn, but 

 only the so-called axis (pseudaxis, Koch). Calcification of the horny 

 interstitial substance has not been made out in Alcyonaria. 



An ectoskeleton occurs only in the tree-like zoanthodeme, and 

 probably also in Pennatulidce. As far as is known, it consists of a 

 secretion of the ectoderm cells of the attached surface ; this secretion 

 increasing with the growth of the colony, forms a horny more or less 

 calcified axis (the sclerobase). Frequently cavities occur in the 

 horny substance, filled either with a spongy material {Gorgonia, 

 Muricea), or by a crystalline substance, rich in calcific matter {Plexau- 

 rella). Often there is an alternation of horny and calcareous lamellae, 

 and in many species the whole axis is formed of alternate horny and 

 calcareous pieces. On the other hand, the horny substance may be 

 uniformly impregnated with calcic carbonate, but spicules never occur 

 in the ectoskeleton. The central part of the axis may remain empty 



