xii THE VOYAGE OF H.M.S. CHALLENGER. 



CHAPTER II.— ANATOMY AND HISTOLOGY, AND EMBRYOLOGICAL NOTES. 



A. Anatomy and Histology. 



We propose in this chapter to treat of the anatomy and histology of the Monax- 

 onida under three principal headings — (I.) the skeleton, (II.) the soft tissues, and 

 (III.) the canal system. This will be found to be on the whole a very convenient 

 division, although not an absolute one, for we shall have to forestall our account of the 

 soft tissues in so far as these are directly concerned with the formation of the skeleton. 

 We hope in this manner to pave the way for a somewhat detailed discussion of the 

 classification which we propose to give in the next chapter. 



I. The Skeleton. 

 (a) The Spicules. 



If any apology be needed for dealing in some bttle detail with a subject which has 

 already received perhaps more than its share of attention at the hands of previous authors, 

 it is to be found in the present state of confusion with regard to the true shapes of some 

 of the minuter and more complex forms of spicules, and with regard to the terminology 

 employed to describe them. Perhaps we may be allowed to say a few words here in 

 self justification. 



The leading, and in fact the only really comprehensive work on the subject published 

 in English is Bowerbank's Monograph of British Spongiadse, and this work leaves the matter 

 in such an unsatisfactory condition that it is absolutely necessary to go over the ground 

 again in order to facilitate a proper understanding of the subsequent pages. Let us see 

 how far these remarks are supported by a brief examination of Dr. Bowerbank's system. 



In the first place we are tokP that " the spicula may be conveniently classed under 

 the following heads : — 1. The essential skeleton spicula. 2. The auxiliary spicula." The 

 auxiliary spicula are subdi \dded into — ' ' Connecting spicula. Prehensile spicula. Defensive 

 spicula. Tension spicula. Retentive spicula. Spicula of the sarcode. Spicula of the 

 ovaries and gemmules." This appears to us an extremely artificial arrangement, more- 

 over it would be very inconvenient in practice, for how can anyone decide whether a 

 given "skeleton spicule" is "essential" or " auxiliary ? " Or how are we to know whether 

 a spicule is a "Tension spiculum" or a "Retentive spiculum'?" We are told that these 

 two classes of spicules are both appropriated to the " membranous tissues of sponges," 

 and that " the ofiice of the first of these is simply to strengthen and support those 



' Mon. Brit. Spong., voL i. p. 13. 



