ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 645 



the retractor muscles. In tlie description of tlio water-vascular system the 

 tentacles are first considered; when these are of different sizes the 

 smaller are always radial and the larger interradial in position. Prof. 

 Ludwig is of opinion that a tentacle is homologous with a pedicel, hut 

 that it is, as a rule, distinguished from it and advanced to a higher grade 

 of development hy the possession of hranches or terminal lohules which 

 are only exceptionally found on the pedicels. The great variations 

 exhibited by the pedicle are described and with them the ambulacral 

 papilla? are discussed. The circular canal and Polian vesicles are next 

 described; the latter vary considerably in nmnber, for while many have 

 only one, Holothuria oxurropa may have from eight to twelve, Chiridota 

 rigicla fourteen to sixteen, and some of the Synaptse have fifty and more. 

 The radial canals are next described and their topographical relations 

 made clear by a diagram of a transverse section through the body of a 

 Holothurian. After describing the tentacular canals and ampulla, and 

 the canals and ampullae of the pedicels, the author commences the 

 description of the stone-canal. 



Formation of Mesoderm in Echinoderms.* — Dr. E. Korschelt has a 

 memoir, based on observations on Strongylocentrotus lividus, with regard 

 to this vexed question. The formation of the layer is a problem of some 

 difiiculty as in all Echinoderms it arises in two ways. It commences 

 with a more rapid grovvth of the cells at the vegetative pole than else- 

 where ; the consequence of this is that at this point the arrangement of 

 the cells becomes irregular ; some are very soon pushed into the blasto- 

 coel. In the course of development there are no signs of any primitive 

 mesenchym-cells. The mesenchym owes its origin to a large number 

 of cells placed at the vegetative pole. The separation of the wandering 

 cells follows no definite law, and in the blastoccel they lie quite 

 irregularly by one another, so that there are not two mesenchym 

 stripes. 



The formation of the mesenchym is very similar in the various 

 groups of Echinoderms, although not in the sense of Selenka— that is, 

 in the presence of primitive mesenchym-cells in all Echinoderms. In 

 the Echinida the mesenchym arises by multiplication of the cells at the 

 vegetative pole of the blastula ; a similar mode is seen in Ophiurids and 

 in some Holothurians (Cucumaria) ; in other Holothurians the mesenchym 

 does not arise till a later stage in development — not, that is, till gastru- 

 lation begins (Holothuria?) or is nearly completed (Synapta), In the last 

 case the mesenchym takes its origin at the tip of the archenteron, as is 

 the case also in Asterids and Crinoids. 



It is possible that we ought to regard as the more primitive form of 

 mesenchym-formation in the Echinoderm that in which the wandering 

 cells break off fi*om the archenteron, rather than their direct origination 

 from the blastodei-m, which may have been more lately acquired. In 

 speculations of this kind, however, we must not fail to remember how 

 little we know as to the phylogeny of the Echinodermata. 



Asteroidea of the Voyage of the ' Challenger.' f — Mr. W. Percy 

 Sladen's bulky memoir deals not only with the Starfishes collected by 

 H.M.S. ' Challenger,' but also those obtained in the expeditions of the 



* Zool. Jahrb., iii. (1889) pp. 653-76 (1 pL). 



t Reports of the voyage of H.M.S. ' Challenger ' (Zoology), xxx. part li. (1889) 

 pp. xlii. and 893, and atlas of 117 pis. and 1 map. 



