8o8 THE ANIMAL KINGDOM. 



pavement cells. He finds (op. cit. p. 16) that the pavement cells here and 

 there in the inhalent canals, in the exhalent, or in both alike, may be swollen and 

 more granular than usual, facts very well seen in Leucetta vera \ The mechanical 

 objections he urges (op. cit. p. 15) against the view that the collared cells are the 

 chief agents in the absorption of food, do not seem well founded, especially if the 

 flagella may be supposed to act in a manner similar to that in which they act in the 

 Choanoflagellata. It may be noted that Krukenberg has extracted a diastatic and 

 peptic ferment from many sponges : the peptic ferment is replaced by a tryptic in 

 very few instances (Sycon =Sycandra, Reniera porosa, 6^.). A piece of raw fibrin 

 laid upon the surface of Suberites massa, S. domuncula, Chondrosia reniformis, 

 underwent resorption : so too in the osculum of S. domuncula. Surface digestion 

 did not take place, however, in Hirdnia variabilis, Spongelia elegans, and Euspongia 

 adriatica. Fibrin was also digested when inserted into the mesoglaea of Suberites 

 massa, but not of S. domuncula. See Krukenberg, Vergleich. Physiol. Studien (i), 

 i. 1881, pp. 65-75 ; and on reserve material in sponges, ibid. ii. p. 42 et seqq. p. 57 ; 

 cf. ibid. iii. p. 113. 



Vosmaer and Heider regard the Porifera as a type in value coordinate with 

 Coelenterata, Polejaeff, Schulze, and others as a sub-type, agreeing with a sub- 

 type represented by the three other Coelenterate classes as far as the gastrula-stage, 

 and then diverging. The group is retained here among Coelenterata for reasons 

 given pp. 715-16; but in the present uncertain state of opinion the value of a 

 class assigned to it must be deemed provisional only. Both Vosmaer and Polejaeff 

 term the Calcarea and Non-Calcarea classes, but it may be doubted whether the 

 essential distinction between them, viz. the hardening of the spicules by calcite or 

 colloidal silica respectively, is a sufficient basis for so great a classificatory dis- 

 tinction. The differences that obtain between some of the orders of Insecta are 

 quite as great. As to the subdivisions of Calcarea and Non Calcarea, they con- 

 stitute in the former a well-defined series, but in the latter their mutual relations, 

 and in the Spiculispongiae and Cornacuspongiae their limits, are by no means 

 established. 



The points to be borne in mind in determining the position of the Porifera 

 seem to be as follows : (i) There are various forms of larvae, the one known as 

 amphigastrula being probably a modified and not a primitive one ; (2) the gastrula 

 is formed either by invagination, or by delamination from cells the common rudi- 

 ment of both endoderm and mesoglaeal cells ; (3) the invaginate gastrula is fixed by 

 its mouth, and the osculum is a secondary formation ; but the gastrula cavity is a 

 true archenteron, and there is no reason for withholding the term ' gastric cavity,' 

 used in a morphological sense, from the various forms assumed by the archenteron 

 in the adults ; (4) the inhalent system of canals is primitively formed by foldings of 

 the surface of the body, and the pores are probably structural adaptations to the 

 mode of fixation ; (5) the increase of the mesoglaea, and the consequent irregularity 

 of form are possibly dependent on the presence of pores ; (6) the skeleton is formed 

 entirely by the mesoglaeal cells as in some Anthozoa Alcyonaria, and is much 



1 It is possible that there are sponges in which there are no ampullae. At least Vosmaer has 

 been unable to detect them in the thin fan-shaped Phakelliae ; see Bijdrag tot de Dierkunde, pt. 3, 

 No. 12, Sponges of the Willem Barents Expedition, 1885, p. 24, Fig. 12. Canals pass from one side 

 of the body to the other in these instances. 



