EEPOPvT ON THE HEXACTINELLIDA. 491 



far as we know, a contrast to the subsequent, and late union of the various spicules in 

 certain Lyssacina. In the latter it is deferred till after the differentiation of the body 

 has made considerable progress, and then only gradually develops from a given centre in 

 the already perfected body. 



But although I regard this difference as most conveniently diagnostic of the two 

 groups, I must allow that it also represents only a qualitative difference. The dictyoninal 

 spicules of the Dictyonina were once, as Marshall first clearly demonstrated for 

 Aulocystis zittelii, free spicules, and the union gradually progresses from the younger 

 to the older portions of the body. We also know of many Lyssacina with connected 

 skeletal framework, like Ehahdodictyum delineatum, 0. Schmidt, Aulocalyx irregidaris, 

 F. E. S., and especially the above-mentioned Euryplegma auricidare, F. E. S., in regard 

 to which we are by no means certain, whether, as in species of Eu2olecteUa and some 

 other Lyssacina, the process of fusion or the formation of synapticula take place some time 

 after the differentiation of the sponge-body, or shortly after its incipient formation. In 

 regard to those Lyssacina in which an abundant fusion of spicules forms a compact base 

 attached to solid bodies, it is very probable that the fusion of the spicules begins somewhat 

 early at the lower end, and progresses gradually upwards to the upper margin. The upper 

 end of many Lyssacina with latticed framework has been long since carefully described, as 

 0. Schmidt has done for his Hertioigia falcifera and other Hexactinellids, and known to 

 consist solely of loose isolated spicules, while the lower end exhibits a well-developed fusion. 



We thus see that the relation at present demonstrable between extant Lyssacina and 

 Dictyonina does not suggest a long-established separation of the two groups, but rather 

 a gradual modification of certain Lyssacine groups into Dictyonina, while others have 

 remained at the undoubtedly older Lyssacine stage. 



We do not, indeed, know the development of the Dictyonina, but it is to be expected 

 that they possess at first only isolated spicules, and recapitulate probably for a short time 

 the Lyssacine stage, before a regular fusion of hexacts occurs, and the typical dictyonal 

 characteristics set in. 



It appears to me further a very noteworthy fact that the results of my bathymetrical 

 statistics show that the Lyssacina occur predominantly in the greater depths and far 

 from the coast in Mid Ocean, while the Dictyonina are chiefly found in moderate depths 

 and near the coast. The Hexactinellids of the Challenger Expedition, which were 

 obtained from the greatest depths belong to the simplest and most typical Lyssacina, 

 as for example certain species of Holascus and Bathydorus, especially Bathydorus 

 Jimhriatus, which was dredged in Mid Pacific from a depth of 2900 fathoms. 



Furthermore, in some species of certain Lyssacine families, such as Euplectellidfe and 

 Eossellidas, we find in the very irregular and long deferred fusion of the larger spicules 

 into a compact connected framework, what may be regarded as an incipient development 

 of a dictyonal skeleton. 



