494 THE VOYAGE OF H.M.S. CHALLENGER. 



very complicated, highly differentiated spicules (discohexasters, floricomes, &c.) occur, 

 and so many species incline towards fusion of the large spicvdes and formation of a 

 connected framework, that one cannot regard these forms at least as primitive. 



A very simple structure is exhibited by certain tubular or saccular Eossellidse, as 

 for instance, Bathydorus Jimhriatus, from the great depths. On the other hand, there 

 are species, like Aulocalyx irregularis, which, both in the formation of a connected 

 supporting framework, and in the complicated structure of the isolated spicules, occupy 

 a decidedly higher grade. Such are the members of the entire division of Cratero- 

 morphinse, in which a more or less firm long stalk is formed, and the chamber-layer 

 exhibits a complicated folding. The Asconematidas exhibit a remarkable affinity ^\-ith 

 the otherwise indisputably far-removed Hyalonematidse, in the possession of autodermal 

 pinuli. Here there can hardly be any direct inheritance, either of one family from the 

 other, or of both from a common ancestor. I am rather of opinion that the tendency to 

 form lateral teeth is very generally distributed among Hexactinellid spicules, and is 

 expressed in development whenever such would be physiologically advantageous or 

 specially useful for the preser\-ation and strengthening of the organism, as for instance 

 in the formation of weapons of capture or defence on the autodermaha projecting from 

 the skin. I believe that the tendency may be expressed in far-removed Hexactinellids, 

 without there being any transmission by inheritance. And this opinion is supported by 

 the fact that, among the typical Scopularia, there are isolated cases in the various genera 

 and species, e.g., in ApJirocallistes and again in Chonelasnm dcederleinii (but not in 

 Clionelasma lamella), in which the distal radial ray of the hypodermalia, projecting more 

 or less beyond the skin, exhibits exactly similar teeth, and thus most distinctly evidences 

 the formation of a pinule. 



In the following sketch of a genealogical tree, including those Hexactinellids which I 

 have investigated, the opinions above expressed are graphically represented. 



When an attempt is made to collate the results of the investigation of li%Tng 

 Hexactinellids -n-ith what is known of fossil forms, an array of difficulties beset the task. 

 Above all it is unfortunate that of the great majority of fossil forms only the dictyonal 

 framework is known, so that there is no possibihty of induction as to the configuration of 

 the soft parts, nor as to the form, number and disposition of the isolated skeletal elements 

 which are so pre-eminently characteristic, and so extremely important, in spite of Zittel's 

 opinion, in determination of relationship. Again, the blanks in the geological record are 

 here even more serious than in most of the other groups. From entire geological 

 formations either no Hexactinellids are known, or only slight traces. Zittel indicates an 

 obvious explanation in the following sentences:^ — "Our knowledge of the fossil 

 Hexactinellids is limited to isolated remains of a developmental series, widely separated 

 (both in time and space) the links of which perhaps lie buried in the deposits now sunk 



> Pateontologie, i. pp. 199, 200. 



