J59^ Royal Society. 



developed close to the posterior extremity of the larva. Delicate 

 sheaves of anastomosing calcareous trabeculse shortly arise within 

 these rings, and the series declares itself as the jointed stem of the 

 pentacrinoid stage, the basal and first interradial plates of the calyx 

 being represented by the already formed casket of calcareous network. 

 The skeleton of the Crinoid is thus completely mapped out within 

 the body of the larva, while the latter still retains its independent 

 form and special organs. 



Within the plates of the calyx of the nascent Crinoid two hemi- 

 spherical or reniform masses may now be detected, — one superior, of a 

 yellowish, subsequently of a chocolate colour ; the other inferior, 

 colourless and transparent. The lower hemisphere indicates the 

 permanent alimentary canal of the Crinoid, with its glandular follicle ; 

 the upper mass originates the central ring of the ambulacral system, 

 with its caeca passing to the arms. The body of the Crinoid is, how- 

 ever, at this stage entirely closed in by a dome of sarcode, forming 

 the anterior extremity of the larva. After swimming about freely for 

 a time averaging from eight hours to a week, and increasing rapidly in 

 size till it has attained a length of from 1 to 2 millims., the larva becomes 

 sluggish, and its, form is distorted by the grovring Crinoid. The 

 mouth and alimentary canal of the larva disappear, and the external 

 sarcode-layer subsides round the calcareous framework of the included 

 embryo, forming for it a transparent perisome. The stem now lengthens 

 by additions of trabeculse to the ends of the joints. The posterior 

 extremity dilates into a disk of attachment. The anterior extremity 

 becomes expanded, then slightly cupped ; the lip of the cup is divided 

 into five crescentic lobes corresponding to the plates of the upper 

 ring ; and finally five delicate tubes, caeca from the ambulacral 

 circular canal, are protruded from the centre of the cup, the rudi- 

 ments of the arms of the Pentacrinoid, At some stage during the 

 progress of these later changes the embryo adheres, and at length 

 becomes firmly cemented to some permanent point of attachment. 



The author states his views as to the morphological and physio- 

 logical relations of the larval zooid. He believes that all the peculiar 

 independently organized zooids developed from the whole or from a 

 part of the segmented yelk in the Echinoderms, and which form no 

 stage in the development of the perfect form of the species, must 

 be regarded as assimilative extensions of sarcode, analogous in func- 

 tion to the embryonic absorbent appendages in the higher animals. 

 For such an organism the term " pseudembryo " is proposed. In 

 the Echinoderm subkingdom, although constructed apparently upon 

 a common plan, these pseudembryos present considerable range of 

 organization, from a somewhat complex zooid provided with elaborate 

 natatory fringes, with a system of vessels which are ultimately con- 

 nected with the ambulacral vascular system of the embryo, with a 

 well-developed digestive tract, and in some instances with special 

 nervous ganglia, to a simple layer of absorbent and irritable sarcode 

 which invests the nascent embryo. The pseudembryo of Comatula 

 holds an intermediate position. It resembles very closely in external 

 form and in subsequent metamorphosis the "pupa stage" of the 



