EEPORT ON THE DEEP-SEA KERATOSA. 87 



was extinct in the cretaceous period, possessed a similar fibrous horny skeleton as in 

 many Halichondriaj. 



Greater difficulties still arise against the modern views on the phylogeny of sponges 

 when we consider the different forms of the canal-system. The two main forms of it, as 

 now generally considered, are the tubular form of the Asconal-type (Asconidse) and the 

 vesicular form of the Leuconal-type (and the allied Syconal and Aplysinal types). The 

 latter, being the more complex, must have originally risen from the former, in which is 

 found the simplest architype of all, the Olynthus, closely allied to the Gastrsea. The 

 great phylogenetical importance of this archisponge, first pointed out in my Monograph 

 of the Calcispongise, is now generally accepted. But the Olynthus there described, and 

 the allied Asconidae, possess a calcareous skeleton ; they must have been derived from an 

 older simple sponge of the same type, which was as yet skeletonless. A slight modifica- 

 tion of this hypothetical Archolynthus seems to be our interesting Ammolynthus. This 

 typical form and the other closely-allied Ammoconidse (Ammosolenia, Ammoconia) supply 

 a new and strong argument in favour of the opinion that the vesicular sponges (with 

 flagellated chambers) originally descended from tubular sponges (with tubular flagellated 

 epithelium) ; we may call the latter (with canal-system of the Asconal-type) Protospongise, 

 the former (with canal-system of the Leuconal-type) Metaspongiee. Starting from this 

 point of view, we may arrive at the following classification of the sponges : — 



First Class. Protospongise, 

 With tubular canal-system (Asconal-type). 



Order I. Ammoconid.e (Malthosa) = Cannoccela. 

 Order II. Asconid.e (Calcarosa) = Homo<xela. 



Second Class. Metaspongi^e, 

 With vesicular canal-system (Leuconal, Syconal, or Aplysinal type). 



Order III. Malthospongi^e (originally skeletonless Keratosa) = Domatoccela (Psam- 

 minidae, and perhaps many Ceratinse). 



Order IV. Demospongle (Sollas, Zool. Chall. Exp., pt. lxiii. p. xcviii). 



Order V. Hyalospongi^e (Hexactinellidse). 



Order VI. CalcispongI/E (Syconidse, Leuconidse, Tichonidse, Pharetronidaj, excluding 

 Asconidae) = Heterocozla. 



