vin CALCAREA HOMOCOELA I 85 



term " triradiate system " (Minchin). The three rays of one of 

 these systems may all be equal in length and meet at equal 

 angles : in this case the system is " regular." Or one ray or 

 one angle may differ in size from the other rays or angles re- 

 spectively, which are equal : in either of these two cases the 

 system is bilaterally symmetrical and is termed "sagittal." A 

 special name " alate " is given to those systems which are 

 sagittal in consequence of the inequality in the angles. Thus 

 all equiangular systems whether sagittal or not are opposed to 

 those which are alate. This is the natural classification. 1 



Sub-Class I. Homocoela. 



The Homocoela or Ascons possess the simplest known type 

 of canal system, and by this they are defined. The body is a 

 sac, branched in the adult, but simple in the young ; its continu- 

 ous cavity is everywhere' lined with choanocytes, its wall is 

 traversed by inhalant pores, and its cavity opens to the exterior 

 at the distal end by an osculum. The simple sac-like young is 

 the well-known Olynthus of Haeckel the starting-point from 

 which all sponges seem to have set out. Two processes are in- 

 volved in the passage from the young to the adult, namely, multi- 

 plication of oscula and branching of the original Olynthus tube 

 or sac. If the formation of a new osculum is accompanied by 

 fission of the sac, and the branching of the latter is slight, there 

 arises an adult formed of a number of erect, well separated main 

 tubes, each with one osculum and lateral branches. Such is the 

 case in the Leucosoleniidae. In the Clathrinidae, on the other 

 hand, branching of the Olynthus is complicated, giving rise to 

 what is termed reticulate body form, that is, a sponge body con- 

 sisting of a network of tubules with several oscula, but with no 

 external indication of the limits between the portions drained by 

 each osculum. These outward characters form a safe basis for 

 classification, because they are correlated with other fundamental 

 differences in structure and development. 2 



As in Halichondria, and in fact all sponges, the body-wall is 

 formed of two layers ; the gastral layer, as we have said, forming 

 a continuous lining to the Ascon tube and its branches. The 



1 Minchin, Lankester's Treatise on Zoology, pt. ii. 1900. 

 - Minchin, loc. cit. p. 110. 



