SPONGES 



such a form as Caulophacus, becomes merged, as it were, in the outer 

 world. The converse series of changes, on the other hand, where, by a 

 process of folding, a portion of the outer world becomes enclosed to form 

 a pseudogaster or false gastral cavity, is not known (pace Lenden- 

 feld) to occur. The osculum of Hexactinellids is typically a wide 

 aperture, frequently partially closed by a delicate sieve-plate (Fig. 18). 

 In Euplectella and its allies (Figs. 15 and 18) parietal gaps, which have 

 no relation to the canal system, occur in the body wall, leading into 

 the gastral cavity. 



2. Skeleton. The skeleton of the Hexactinellid sponges is of 

 great interest from the morphological point of view, since the 

 spicules exhibit in remarkable manner the persistence of one funda- 

 mental type in the midst of infinite variations. 



Forms of the Spicules. The primitive type of spicule in the 

 Hexactinellids is the regular hexactine, a form with six similar and 

 equal rays meeting at right angles at a common centre (Fig. 47, e). 

 Each ray is traversed by an axial organic thread, which after 



FIG. 81. 



Modifications of the triaxon type of spicule. a, sword-like hexactine ; 6, c, two varieties of 

 the pinulus ; d, amphidisc ; e, pentactine ; /, tetraetine ; g, rhabdus. 



maceration becomes a minute canal. The six axial threads meet at a 

 point, forming the so-called axial cross, a structure of great importance 

 for determining the morphological centre of the spicule. 



Spicules of this form are of common occurrence in most species 

 of the group. More commonly, however, the primitive hexactinal 

 form has become diversified by modifications, which may be grouped 

 into two series. 



In the first place, one or more of the rays of the primitive 

 hexactine may vary in size relatively to the other rays, so as to 

 become either greatly hypertrophied, on the one hand, or reduced 

 even to the vanishing point, on the other hand. Unequal develop- 

 ment of the rays results in peculiar forms of the hexactine, such as the 

 sword-like hexactines, characteristic of the Eupledellidae (Fig. 81, a). 

 Complete atrophy, or rather arrested development, of one or more 

 of the rays, causes the primitively six-rayed type to become pent- 

 actinal, tetractinal, and so on, until finally only one or two rays 

 remain (Fig. 81, <?, /, g), and as the end term of this series we have 

 a simple monaxon rod, which may be either diactinal (rhabdus), or 

 monactinal (style). So long, however, as there are more rays than 



