BLASTOIDEA. 135 



Billings dissents from this view. The remaining five apertures 

 are placed at the summit of each deltoid plate, between two 

 pseud-ambulacra. Four of the apertures are equal in size, 

 and have generally been regarded as " ovarian ; " but they are 

 looked upon by the above-mentioned eminent authority as 

 being connected with the respiratory system. The fifth is of 

 larger size than the other four, and is usually regarded as 

 partly ovarian and partly anal ; but it is looked upon by Mr. 

 Billings as " oro-anal." 



Like the Cystideans, the Blastoidea are not only extinct, 

 but are exclusively Palaeozoic. The Cystideans, however, 

 attained their maximum at a very early period of Palaeozoic 

 time; whereas the Pentremites flourished most abundantly in 

 the Carboniferous seas, towards the close of the Palaeozoic 

 period. The Cystideans had nearly died out, or had become 

 quite extinct, by the close of the Upper Silurian period ; and 

 it is a noteworthy fact that it is just at that point that the 

 Blastoidea seem to make their first appearance. Very spar- 

 ingly represented in the Upper Silurian Rocks, the Blastoidea 

 increase largely in numbers during the Devonian period ; but 

 they attain their maximum of development in the seas of the 

 earlier portion of the Carboniferous period. With one ex- 

 ceedingly problematical exception, no member of the order 

 is known to have survived the close of the Carboniferous 

 epoch. 



ORDER VII. HOLOTHUROIDEA. 



The last order of the Echinodermata is that of the Holothu- 

 rians or " Sea-cucumbers," in which the body is vermiform or 

 slug-shaped, and the calcareous matter secreted by the integument is 

 reduced to scattered spicules (Synapta), or rarely is present in the 

 form of imbricated scales (Psolus). 



As might have been expected from the generally soft nature 

 of their integuments, the Holothuroids are hardly known as 

 fossils, and they merely require to be mentioned here. The 

 only remains referred with any probability to this order are 

 certain calcareous spicula which have been found in deposits 

 of both Mesozoic and Tertiary age, and which have been re- 

 garded as belonging to a Synapta, and the shield of a species 

 of Psolus which has been found in Post-Tertiary deposits in 

 Bute. 



