﻿SPONGES.^HALLMANN. , ^-, 



SuB-FAMiLv MVXILLIN.4i;. 



i he preponderance of evidence is in favour of the supposi- 

 tion that the most primitive Myxillinae possessed a type of 

 organisation closely resembling that which obtains in certain 

 existing genera such as Leptosia, Hymermncom, Hymetro- 

 chota and Hymeraphia ; and it is possible, in accordance there- 

 with, to imagine a common ancestral form, or hypothetical 

 "Promyxilline," characterised by the following features : — The 

 sponge grew in the form of a thin encrustment, and produced 

 in contact with the substratum a basal layer of spongin echi- 

 nated by erect acanthostylote spicules which acted as pillars 

 for the support of the soft structures. These spicules (which 

 for convenience will be termed the basical megascleres) ex- 

 hibited a tendency to differentiate into two kinds, a less spiny 

 larger, and a more spiny smaller kind, and this was probably 

 associated with an accompanying tendency towards an ar- 

 rangement of the spicules in clusters, in each of which larger 

 individuals were surrounded by smaller. Megascleres of a 

 second kind, smooth and probably monactinal, were also 

 present, these (which may be distinguished as auxiliary'^ 

 megascleres) occurred, without definite arrangement, more 

 particularly in the superficial layers of the "sponge, and also 

 formed descending strands stretching towards the sponge- 

 base. Sponginous fibres were not produced, but the pri- 

 mordia of such, in the form of low dome-shaped elevations 

 of the basal spongin lamina, coinciding in position with the 

 areas occupied by the spicule-clusters, had probably made their 

 appearance. It is diflicult to say what the microscleres were, 

 but since cheloids, sigmata, toxa and rhaphides (often in drag- 

 mata) are found in the group, it is necessary to assume that 

 these, or at least the forms from which they have been derived, 

 were present. 



From such a hypothetical form all the different types of 

 Myxillina" are capable of being derived. 



The evidence afforded h\ the microscleres, considered in 

 conjunction with other facts, indicates either that a consider- 

 able amount of evolution in various directions had been ac- 

 complished, and that the prototypes of quite a number of 

 different groups of co-related genera had already come into 

 existence prior to the origin of sponginous fibres and to the 



1 These spicules are commonly known as "dermal" or "ectosomal" 

 megascleres; but the part which they play in the conformation of the 

 skeleton varies to such an extent in different grenera that it is advisable 

 in a comparative treatment to designate them by a term unsuggestive of 

 position or function. The term "auxiliary" is not altogether an appro- 

 priate one, but it will suit the present purpose; the spicules to which it 

 applies correspond for the most part, in the MyxilliuEe, to the megascleres 

 which Bowerbank included under the same name. I shall apply the 

 epithet "dermal" only to those spicules, of whatever category, which 

 are specially concerned in the formation of a dermal skeleton. 



