372 



COMPARATIVE ANATOMY 



CHAP. 



of the wedges regularly alternate. Or again, the joints may be arranged in two series 

 or rows, the contact-surfaces of the one row alternating with those of the other, and 

 the two rows themselves interlocking along a zigzag line (biserial). 



The Articulata, many Cfind/iculnt/i, and the recent Li'iiliinnta have the joints 

 of their arms arranged in single rows. This condition has been proved to be 

 ontogenetically and phylogenetically primitive, i.e. for the palaeozoic L\ilii,nii 

 and the Camerata. The majority of palaeozoic Inadunata have uniserial arms, 

 but towards the end of the palaeozoic period forms appeared with alternating rows 

 (e.g. Pofcriocrinus), and finally some genera in which the brachials may be biserial 

 at the tips of the arms (/>//</< //o-///.s-. AV/.y/c/-///"*, Bydretonocriwtu). 



Most of the Caiii'Tiitu (an order limiti'd to tin.- paleozoic age) have biserial 

 arms. But by far the greater number of the Lower Silurian species have uniserial 



I 'n;. 3-23. Part of the arm 

 of a Crinoid. Diagram showing 

 tin- transition from the uniseriul. 

 through the alternating, to the 

 biserial arrangement of the 

 brachials. 



Fi<;. 3.'4. Part of the disc formed by the arms 

 of Crotalocrinus rugosus (after Wachsmuth and 

 Springer). ~J, The trabecuhe connecting tin: anus; 

 In-, the anus with the covering plates (<;') over their 

 fond grooves ; in 3 these covering plates are removed. 



arms. In the Upper Silurian, however, but few forms persisted with such arms. 

 and they are found side by side with species and genera with alternating, or with 

 two rows of, brachials. 



In Crinoids whose arms have two rows of joints, the uniserial and the alternate 

 stages are passed through ontogenetically. It must, further, be specially emphasised 

 that not a single case is known of arms being formed of two rows of brachials 

 throughout their whole length, i.e. from the radials of the calyx to their tips. At 

 their bases the arms always, for a certain distance, have a single series of brachials, 

 then they have alternating brachials, and finally two rows. The transformation of 

 the uniserial arm into an alternate, and finally into a biserial one commences, 

 ontogenetically and phylogenetically, at the tip of the arm, and proceeds from that 

 point towards the base. 



The food grooves of the arms resemble those of the calyx. They are sometimes 

 naked and open, and at others provided with a variously developed ambulacral 

 skeleton, consisting either only of lateral plates, or of lateral and covering plates. 

 Subambulacral plates may also occur in the floor of the food grooves, dividing them 

 from the subjacent organs of the ambulacral furrows of the skeleton (body cavity of 

 the arms, genital strands, pseudohremal canals, etc.). Where covering plates are 

 present there are two rows which alternate and interlock in such a way as to form a. 



