252 DESCRIPTIONS OF PREPARATIONS. 



water-canal or into a system of 'subdermal spaces ' (A : s.s.). The afferent canals 

 (A and B : a.c.) branch, and penetrate the substance of the sponge. The ultimate 

 branches, -01 mm. in diameter, open two to four together, into the globular portion 

 of pyriform ampullae or ciliated chambers (A : a., B.). The stalk of the pear opens 

 either directly into an efferent water-canal (A and B : e.c.} or into one of its root- 

 lets. The branches of the efferent canals unite inter se, and finally open into an 

 oscular canal. This canal is straight, two to three mm. wide, and rises verti- 

 cally to the surface, where it opens by an osculum, which is limited by an iris-like 

 membrane. 



The sponge-substance is composed of three tissues : ectoderm, endoderm, and 

 mesodermic tissue. The ectoderm consists of flat nucleated cells covering the 

 outer surface. It is not certain whether the cells coating the subdermal spaces and 

 afferent water-canals belong to the ecto- or to the endo-derm. They appear at any 

 rate to absorb solid particles and hand them over to the cells of the mesodermic 

 tissue. The ampullae are lined by cylindrical granular endoderm cells about 

 sixty to each ampulla with a basal nucleus, single cilium surrounded by a hyaline 

 protoplasmic collar (B : c.c.}. The currents of water which traverse the sponge are 

 due to the motion of these cilia. The efferent water-canals and the oscular canals 

 are lined by flat cells (B : ep.\ which are endodermic apparently in origin. The 

 cells near the mouth of an ampulla have frequently a transitional character. The 

 mesodermic tissue is present in mass. It consists of a matrix (A and B : m.) 

 hyaline save in the region of the ampullae, where it is opaque from the presence 

 of many rounded granules. It imbeds (i) stellate (C : s.c.) or fusiform connective 

 cells with processes which appear to anastomose ; (ii) cells containing pigment, 

 most numerous near the surface of the sponge and round the large afferent canals ; 

 (iii) cells devoid of all processes, which often contain globular masses of a refractile 

 substance, fatty or starchy in nature (?) ; (iv) long fusiform contractile fibre-cells, 

 either isolated or in bundles, on the surface of the sponge, or disposed circularly in 

 the annular constrictions of the water-canals, and the iris-like membrane of the 

 oscula, both parts which show contractility ; (v) the fibrous skeleton. This skeleton 

 consists of thick principal fibres (A :/.), which radiate from the base of attachment 

 and end with fine points (A : _/".) in the conuli, and of more delicate branched 

 secondary fibres (A : /".), which connect the principal fibres. Each fibre consists of 

 a central granular axis surrounded by hyaline strongly refracting lamellae of Keratin 

 or Spongin, a substance near akin chemically to silk, and which in most instances 

 polarises light. The principal fibres have a rough exterior, and their axes contain 

 foreign bodies, chiefly sand-grains, apparently taken in by the mass of irregular cells 

 surrounding the tip of the fibre wherever the fibre is lengthening. The hyaline 

 lamellae are secreted by pear-shaped cells or spongoblasts (C : sp.\ which are prob- 

 ably modified connective tissue or mesodermic cells. The secondary fibres arise 

 independently, and become connected subsequently to the principal fibres. 



Euspongia may be propagated artificially from fragments. The sexes appear 

 to be separate, and the male sponge rare. The sexual elements, derived probably 

 from mesodermic cells, undergo development in spaces of the mesodermic matrix 

 lined by flat cells. The ripe ovum is -25 mm. in size, is impregnated and developed 

 into a larva in situ. The free larva is -4 mm. long, -35 mm. broad, shaped like a 

 conical bullet and composed of an external layer of ciliated and pigmented cells, 



