542 INVERTEBRATE MORPHOLOGY. 



than as a distinct stalk, while iii other forms, such as Antedon 

 aud Actinometra, the stalk, though present in young forms, is 

 entirely wanting in adult life, during which the animal is free- 

 swirnming, though having the power of anchoring itself 

 temporarily to solid objects by means of a number of slender 

 processes termed cirri which project from the apex of the cup 



(Fig. 251, c). 



The lower portion of the cup, or calyx, is formed by a num- 

 ber of series of calcareous plates united to each other by 

 sutures, while its mouth is covered in by a flat or dome- 

 shaped disk in which calcareous plates may or may not be 

 present. In the centre of the disk is the mouth of the animal, 

 while to one side is the anus, lying in the interradius CD. 

 From the mouth five grooves, known as the ambulacra! grooves, 

 extend outwards towards the margin of the cup, and, near the 

 margin, branch, being then continued outwards on the oral 

 surfaces of ten arms which arise from the junction of the disk 

 and calyx, frequently branching in their course, and bearing 

 along their sides a series of short processes resembling them 

 in structure, and termed the pinnules, upon which the ambu- 

 lacral grooves are also continued. These arms are capable of 

 considerable movement, being at one time extended out at 

 right angles to the body or even reflexed, and at another 

 coiled up circinnately over the disk, the pinnules being at the 

 same time bent inwards towards the median axis of the arm. 



The stem when present consists of a number of disklike or 

 cylindrical calcareous plates, placed one on top of the other, 

 being held together by bauds of connective tissue, and is 

 traversed by a central canal containing prolongations of certain 

 of the visceral structures. The terminal plate serves as the 

 point of fixation, the plates immediately above it having 

 attached to them a number of cirri which assist in fixation 

 and are, like the stem, composed of calcareous plates contain- 

 ing prolongations of the central canal. In some forms, such 

 as Pentacrinus, whorls of cirri also occur at intervals all along 

 the stem, those plates from which they arise being termed 

 nodal plates, a varying number of plates destitute of cirri 

 occurring between two nodes in different genera. In certain 

 genera, however, such as Hyocrinus and Rhizocrinus, these 



