402 ECHINAXIA AND RHABDOSIGMA, 



their plumose arrangement is generally the more pronounced, 

 towards the outermost extremities of the extra-axial fibres {i.e., 

 near the surface of the sponge). The connecting fibres contain 

 only one or two spicules. A most unusual (possibly abnormal) 

 circumstance in connection with the coring spicules of the axial 

 main fibres, — apparently, however, only in the older portions of 

 the skeleton, — is the fact that a variable (often a very consider- 

 able) proportion of them fail to grow much beyond their very 

 earliest developmental stages, — their immaturity in many cases 

 being such that nothing more of them can be discerned than the 

 faint outline of their axial canal. The echinating spicules are 

 usually directed almost or quite perpendicularly to the fibres, 

 and stand, on the average, at a distance apart of (at the most) 

 not more than one-half their own length. A considerable pro- 

 portion, if not the majority, of the echinating spicules project 

 from their supporting fibres in such directions as apically to 

 impinge upon and become inserted into adjoining fibres, and 

 thus come to play the part of connecting spicules; in the axial 

 region of the skeleton, and also (as already mentioned) in the 

 older portions of the extra-axial region, these connecting spicules 

 usually become entirely ensheathed in spongin, and thus give 

 rise to additional connecting fibres. 'I'he extra-axial main fibres 

 are usually between 60 and 80/x, rarely as much as 100/x, in 

 stoutness; the axial main fibres, owing to the greater develop- 

 ment of their spongin-sheath, occasionally attain to over 250/x 

 in stoutness in the stalk of the sponge, but in the lobes seldom 

 exceed 150/x. The terminal spicules of the extra-axial main 

 fibres are free from spongin except basally, and form, at the 

 extremity of each fibre, a slightly divergent penicillate tuft, pro- 

 jecting into the dermal membrane. There is no dermal skeleton. 

 Spicules. — The rhabdostyli (Text-fig. 2) range from 160 to 280/x 

 in length and attain a maximum stoutness of 18yu; in their very 

 earliest developmental stages they already measure 5 or 6/x in 

 stoutness, and the diameter of their axial canal is only slightly 

 less. They have the shaft bent, as a rule rather sharply and 

 angulately, at a distance of from 20 to very seldom more than 

 30/Ji from the base, and at an angle usually much less than 



