244 PROFESSOR ALLMAN ON A PRE-BRACHIAL STAGE 



him, when he made his drawing, the little animal Avhich forms the subject of the 

 present paper, or at all events, the same phase of a nearly allied species ; but his 

 figure, while it gives a better idea of the form of the roof than Thomson's, throws 

 no light on the composition of the calyx, and is in other respects so deficient in 

 detail as to render it of little use in working out the relations and significance of 

 the little Crinoid. 



Taking for granted, then, that the subject of the present paper is a pre-brachial 

 stage in the development of Comatula, it will hold an intermediate position be- 

 tween the free larvae described by Busch,* and the fixed Pentacrinus stage 

 described by J. V. THOMrsoN.f 



A comparison of our larva with the adult Comaiula is full of interest. In 

 order, however, that this comparison may be made with advantage, it will be 

 necessary, in the first place, to endeavour to assign their true value to the several 

 plates which enter into the composition of the body- walls of the larva. 



Now, the narrow zone which intervenes between the five large hexagonal 

 plates and the summit of the stem, corresponds undoubtedly to the centro- 

 dorsal piece which, in the adult Comaiula carries a set of articulated appendages. 

 Whether this piece is to be regarded as the homologue of the true basilar zone in 

 the typical Crinoidea, or rather as a modified superior joint of the stem, is a point 

 not immediately obvious. I entirely agree, however, with Johannes Muller, in 

 viewing the centro-dorsal piece in Comatula as a metamorphosed stem joint ; for 

 not only do we fail to detect in this portion of the calyx a composition out of 

 distmct plates, but, what is of more importance, it is from this very piece that 

 the articulated appendages are developed,— these being, as shown by the analogy 

 oi Pentacrinus, properly appendages of the stem. 



If this be the true interpretation of the part in question, then the five large 

 plates, which are superimposed upon it in our larva, and which here constitute 

 almost the whole of the calyx, will correspond to the true basalia, which imme- 

 diately surmount the stem in such forms as Platycrinus, and the great majority of 

 the Crinoidea, while the five small plates intercalated between their upper angles 

 will represent radialia; for it is manifest, as shown by the larval Comatula in a 

 more advanced stage — for a preparation of which I am indebted to tlie kindness 

 of Dr Carpenter — that it is in the position of these little plates that the proper 

 arms will subsequently arise. 



* Busch, "BeobachtungeniiberAnat. und Entwickel. einigerWirbelosen Seethiere." Berlin, 1851. 



\ Thompson, loc. cit. Since reading the present paper, I met with an abstract of a paper on the 

 " Embryogeny of Comatula rosacea" by Professor Wyville Thomson, in the " Proceedings of the 

 Royal Society of London" for Feb. 5, 1863. In this paper Professor Thomson gives us the results of 

 a sei'ies of valuable observations in which he has traced the development of the larva beyond the stage 

 at which it was left by Busch, and has further elucidated its earliest stages. He has not, however, 

 ean-ied it up to the point which it is the object of the present paper to describe ; while the very 

 careful and complete researches of Carpenter, referred to below, would seem to start from a point a 

 little in advance of that here described. 



