Rev. T. Hincks o^i the History of the Hydroida. 385 



that plate 2 also is so dilated that it enters into this circle ; and 

 this increase indicates that when of a less size than 6"5 millims. 

 MeUita may have all its ambulacra separate. This is actually 

 the case in Echinarachnius parma. A young individual of 

 this species, 6'5 millims. in length, has all live ambulacra 

 similar ; and their plates 2 are not yet broad enough to affect 

 the form of the interradia, all of which are free and connected 

 in uninterrupted sequence. But at a length of 34 millims. 

 plates 2 of 1. a and V^. h in the bivium have so increased as to 

 touch each other and internipt the unpaired interradium, 

 although one of the plates 2 is excluded from contact with 

 plate 1 still only in interradia 1 and 4. It follows from this 

 that the regular form, with five similar ambulacra and five 

 similar interradia, is the primordial one in the Clypeastridge, 

 which is retained by Echinocyamus and Layanum^ but from 

 which Encope, Clypeaster, and Stolonodypus^ Mellita and 

 Rotiilaj Echinarachnius and Lohophora^ together with Arach- 

 ■noidesj depart during growth, the five last named, moreover, 

 forming a bivium. Pressure towards the peristome during 

 growth and the addition and great multiplication of new 

 plates in the petala which are destined for the branchise have a 

 great part in these changes. The " equator " is not the same 

 during the whole life of the animal. We may see, by com- 

 paring several Echinarachnii of difterent ages, how a great 

 part of the plate through which the periphery passes gradually 

 goes over to the ventral surface, until the following one becomes 

 visible there, and the periproctium, which is at first dorsal, 

 becomes finally more than half ventral. At the same time the 

 stoma of the test, as in the Echini, becomes less in proportion 

 to the whole animal. In a Mellita hexapora 6 millims. broad 

 the transverse diameter of the stoma is about 0T3 of that of 

 the disk, at 8 millims. O'l, at 35 millims. 0*04, and at 80 

 millims. 0*034. 



[To be continued.] 



LIV. — Contributions to the History of tJie Hydroida. 

 By the Eev. Thomas Hincks, B.A., F.Ii.S. 



I. TJie Sarcothecce [Nematophores) of the Plumulariidge. 

 [Plate XX. figs. 1-3'.] 

 The singular organs to which the name nematophore has been 

 assigned by Busk, and which are confined to the Hydroid 

 family of the Plumulariida?, have been investigated by several 

 able observers ; and much light has been thrown on their struc- 

 tural and physiological history, though as yet little is known 



