EEPORT ON THE KERATOSA. 47 



quite devoid of any enclosed corpuscles. As to the foreign enclosures themselves, they 

 are represented in both the Challenger specimens almost exclusively by sand-grains, some 

 of Dr. Marshall's specimens having proved to be also very rich in them, while others, on the 

 contrary, were poor in sand and rich in fragments of mussel-shells. I agree entirely, how- 

 ever, with Dr. Marshall as to the impossibility of paying any systematic attention to such 

 differences. Of course the faculty has been ascribed to sponges of choosing from the avail- 

 able foreign bodies those which they need. Haeckel adopts it with respect to his Physe- 

 maria;'^ and Carter, though on a different occasion,'' speaks also of " that developmental 

 intelligent power whose existence in every organised product is only known to us by its 

 manifestations." However, the contrary opinion, held by F. E. Schulze and Marshall, is 

 supported by more valid arguments, and there is absolutely no necessity for introducing 

 into our scientific calculations a new thoroughly unknown factor, while the phenomenon 

 admits of a very simple and plausible mechanical explanation. 



So far now as the anatomy of the form in question is concerned, Marshall believes the 

 Psammopemmata to be of very low organisation, and if I understand him aright, he sees 

 it in their lipostomy and lipogastry. These two peculiarities, provided that the lipostomy 

 be really characteristic of the genus, are, however, of very subordinate significance ; a 

 lipostomic and lipogastric Leuconid is yet more highly organised than a Sycon provided 

 with the broadest central cavity and with an osculum fringed with the most elegant 

 spicules, — such questions, without knowledge of the structure of the canal system, not 

 being at all capable of solution. That of the Psammopemmata agrees in its features 

 closely with that, for instance, of Sp>ongelia pallescens, as described by F. E. Schulze. 

 The flagellated chambers are large, of more or less regularly roundish outlines, and devoid 

 of any special cameral canaliculi ; the ground-mass surrounding them is transparent and 

 without granules (PI. III. fig. 4). There is also a close resemblance to the representatives 

 of Spongelia in the histological properties, the only difference being that such aggrega- 

 tions of fusiform cells as Schulze ^ describes for Spongelia avara are not to be found 

 in the two Challenger specimens of Psammopemma. In this statement I differ from 

 Marshall, who lays stress on their constant occurrence in (under ?) the covering dermis. 

 I find these fusiform cells scattered everywhere and also under the dermal membrane, 

 but in most cases lying isolated, and never in such mutually parallel disposition as in 

 Marshall's illustration {loc. cit., pi. viii. fig. 10). Nor can I agree with him as to this 

 dermal membrane being homogeneous {loc. cit., p. 113). I found it to contain nuclei 

 tlisposed at approximately equal distances from one another, and on the ground of 

 numerous analogies I am inclined to regard it as a common pavement-epithelial layer, 

 the boundaries of its separate ceUs having disappeared owing to the preservation in 

 alcohol. 



1 Biolog. Studien, Heft. ii. p. 213. 



2 I refer to his discussion of the process of the horuy skeletal fibres taking in and enclosing the foreign bodies, 

 Ann. and Mag. Nat. Hist., ser 5, vol. viii. p. 113. ' Zeitschr.f. wiss. Zool., Bd. xxxii. p. 136. 



