136 Mr. W. J. Sollas on the 



fined to the upper corners of the crypts, to which they serve as 

 a kind of groin ; the arms of large trifids sometimes occur in 

 the same position, and where both are absent their place is 

 sometimes supplied by a projecting spur produced from the 

 shaft of one of the robust trifids (PL VI. fig. 8). 



Stellates. — The cylindro-stellates are most abundant in the 

 dermal layer, where the sharp-rayed forms are rare ; both 

 kinds of stellates are thickly strewn around the walls of the 

 crypts, and both are rare in the interior of the mark. The 

 sharp-rayed forms preponderate in the mark. 



Trichites. — The trichite sheaves occur as the chief consti- 

 tuents of the outer half of the cortex ; they accompany the 

 large spicules through the pillars of the crypts, and are abun- 

 dantly dispersed throughout the general substance of the mark. 



Foreign Bodies. — The mark contains a surprisingly large 

 number of foreign bodies imbedded in its substance. The nature 

 of these included bodies is very various ; but, for the most 

 part, they consist of tests of Foraminifera, Radiolaria, and 

 Diatoms, and the calcareous and siliceous spicules of a variety 

 of other sponges, including an occasional Geodia globule. 



Observations. 



1. The Muscular Layer. — Before proceeding to describe this 

 a little more fully than we have yet done, it may be worth 

 while giving a short account of observations which have been 

 already made by others on the occurrence of muscular tissue 

 in the sponges. 



Lieberkilhn * appears to have been the first to draw atten- 

 tion to the resemblance between certain sponge-tissues and 

 unstriated muscle-fibre, as in his description of the fibrous 

 layer of the cortex in Tethya lyncurium, where he says that 

 the fibres of this layer may be regarded as related to the 

 so-called organic muscle-fibre of the higher animals. 



Oscar Schmidt f follows, quoting Lieberkiihn, confirming 

 his observations, and extending them to other species, ex. gr. 

 Ancorina cerebrum, Sdt. 



Kolliker % likewise describes the muscular tissue of certain 

 rind-sponges. 



O. Schmidt § again discusses this subject, confirming, by 

 his own observations on the intermarginal cavities of Geodia 

 gigas, Sdt., those made by Bowerbank on his Geodia Baretti, 



* Leiberkiihn, 1859, Archiv f. Anatomie u. Physiologie, p. 523. 

 t O. Schmidt, 1862, Die Spong. d. Adriatischen Meeres, p. 43, pi. iv. 

 fig. 1, a, b. 



I Kolliker, 1864, Icon. Histolog. i. Heft, p. 48. 



§ O. Schmidt, 1866, Adriat. Spong. ii. Supplem. p. 3. 



