62 SUMMARY OF CURRENT RESEARCHES RELATING TO 



formed near the middle line of the under side of the ray, and grow 

 towards the peripheral end ; the first formed are the adoral, and these 

 bifurcate in the neighbourhood of the median line. 



The first interbrachials, which are regarded by Mr. Fewlies as the 

 odontophores, originate as heart-shaped, interradially placed calcifica- 

 tions, five in number ; each is placed abactinally to the interbrachial 

 ends of the oral ambulacrals. 



No ventral embryonic row of spines was observed in any species of 

 Asterias which was studied. 



The author regards the genitals of Asterias as homologous with the 

 " basals " of AmpMura ; the first interbrachial is homologous with the 

 orals of Amphiura ; the madreporic opening is placed on homologically 

 diiferent plates in Asterias and Amphiura. The interambulacrals of 

 Asterias are the homologues of the laterals of Amphiura. The oral 

 ambulacrals of the former are represented by the " spoon-shaped " plates 

 of the latter. The first and second adambulacrals have no homologues 

 in the mouth-parts of Asterias. The dorsolaterals and the connectives 

 of the arms of Asterias were not recognized in Amphiura. There is some 

 doubt as to the homologies of the marginals. 



Development of Synapta digitata.* — Dr. E. Semon has made a 

 careful examination of the development of this Holothurian. Segmen- 

 tation is remarkably equal. While in Echinids and Ophiurids the forma- 

 tion of mesenchym precedes the invagination of the archenteron, in the 

 Holothurians it succeeds it ; it is not possible to decide which of these 

 two is the more archaic arrangement. The ciliated bands of the 

 Auricularia-lavya are local thickenings of the ectoderm ; the other 

 ectodermal cells simultaneously lose their flagellaj^and become flattened. 

 The somewhat remarkable fact that the larvae of Asterids have two, and 

 not, like other classes of Echinoderms, only one ciliated band, is ex- 

 plained by the discovery of an adoral band, from which the second 

 circlet is developed. It may be concluded that all bilateral echinoderm- 

 larvse have two separate ciliated bands, one adoral and one postoral ; and 

 there is no essential difference between the larvae of Asterids and those 

 of other classes; the characters of the larvae are discussed at some 

 length. 



Flattened mesenchym-cells form a simple and not completely con- 

 tinuous layer beneath the epidermal investment, and form a half-groove- 

 like sheath to the ciliated bands and the stripes of the lateral surfaces, 

 as well as an investment for the stomach and rectum. These cells are 

 very much flattened, and are thereby distinguished from the other mesen- 

 chym-cells. The larva has, at an early stage, an extremely thin 

 epidermis, which is formed by the ectodermal cells which were at first 

 ciliated, and a unilaminate cutis which is formed of mesenchym-cells. 

 There is no doubt that the two bands discovered by Metschnikoff are the 

 nervous system of the larva ; this is shown, not only by the whole struc- 

 ture of the organ, but by the fact that, later on, the bands pass into the 

 permanent nervous system of Synapta. 



When the larva enters the Auricularia-stage the rudiment of the 

 hydroenterocoel is a simple elongated vesicle, which opens to the exterior 

 by the dorsal pore. At first it lies in about the median plane of the 

 larva, and later on it passes to the left side. Jt next divides into two 



* Jenaisch. Zeitschr., f. Naturwiss., xxii. (1888) pp. 175-S09 (7 pis.). 



