428 [Feb. 5. 



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 lengh of from 1 to 2 millims., the larva becomes 

 sluggish, and its form is distorted by the growing 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 perisom. 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 

 Holothuridse, the great distinction between them being that in the 

 Holothuridse the pupa has already passed through the more active 

 "Auricularian" stage, while the analogous form in Comatula has 

 been developed directly from the egg. 



