GENERAL PART. 61 



structure. The ciliated bands are well developed and the larva 1 , on leaving 

 the marsupium, must swim a little distance. This step is indeed not so great 

 from the usual case in Comatulids, the embryos being kept on the pinnules 

 until they are nearly ready to attach and, probably, in many cases simply 

 sinking down from the pinnules to the cirri to attach themselves there. 

 Indeed, it may be said that care of the brood is the normal fact in Comatulids, 

 Tropiometra and Antedon pctasus being the only known exceptions. If it were 

 the usual condition in Comatulids that the eggs were free (or pelagic, as in 

 Tropiometra) and then a case was discovered where the eggs and embryos 

 remained attached to the pinnules until the fixation stage is reached, then 

 no doubt anybody would designate that as a case of care of the brood. 40 The 

 three "viviparous" Crinoids named above accordingly represent only further 

 steps in the viviparous habit common to most Comatulids, the formation of a 

 special marsupium being the most essential new feature. In Isometra the 

 larvae are not very much influenced thereby. Notocrinus represents a much 

 more specialized case, the larvae having lost their ciliated bands and probably 

 never leaving the marsupium. At any rate, this is the case in Thaumatometra 

 nutrix, where the Pentacrinoids remain attached to the pinnule, just protrud- 

 ing the head through the opening of the marsupium. The same is probably 

 the case in Notocrinus. Unfortunately the larva of Thaumatometra nutrix is 

 unknown, but it may be expected that it will prove to have lost the ciliated 

 bands, as has the larva of Notocrinus. 



2. THE CLEAVAGE. 



That the total regular cleavage, as found in Antedon and Tropiometra, will 

 prove to be of general occurrence among the Comatulids there is no reason to 

 doubt. The remarkable meroblastic cleavage of Isometra vivipara is evidently 

 an exceptional case, due to the special conditions afforded by the viviparous 

 habit of this species. It may be reasonably expected that a similar mero- 

 blastic cleavage will be found to occur also in Notocrinus and Thaumatometra 

 nutrix, where the conditions for the developing egg are similar. Upon the 

 whole, viviparity in Echinoderms evidently has some connection with the 

 meroblastic type of cleavage. It has been observed only in viviparous 

 Echinoderms, namely, besides Isometra vivipara, in the following species: 

 Cucumaria glacialis Ljungman, Amphiura vivipara H. L. Clark, Hypsiechinus 

 coronatus Mortensen, Abatus cavernosus (Philippi), and Amphipneustes koehleri 

 Mortensen, and it can hardly be doubted that it will be found in many other 

 viviparous Echinoderms, though not in all of them; in Amphiura squamata 

 the cleavage is total, though irregular, and it is also total and quite regular 41 

 in Synapta vivipara [Synaptula hydriformis (Lesueur)]. The eggs of the 



40 In fact, Ludwig in his paper "Brutpflege bei Echinodermen " (Zool. Jahrbiicher. Suppl. vii, p. 699, 

 1904, counts Antedon rosacea among the Echinoderms which protect their brood. 



41 H. L. Clark. Synapta vivipara: a contribution to the morphology of Echinoderms, Mem. Boston Soc. 

 v, p. GO, 1898. 



