V. Echinoidea. 17 



parts of one and the same arm which had been broken off from its disk and had 

 developed a new one in the middle of its length, from which the three small arms 

 had grown. Lildwig ( 4 ) shows the impossibility of this having occurred and ex- 

 plains the condition as a case of regeneration in an individual which had lost the 

 periphery of its disk, together with three arms and the distal parts of two others. 



Gregory (*) describes a new Protaster from Australia with the arms composed 

 of bluntly oval ambulacral ossicles, which are unusually thick, and placed alter- 

 nately in the concavities of a sinuous medio-dorsal ridge. It has more resemblance 

 to Brisinga than any Protophiuroid yet known. 



Boehm has studied a number of fossil Ophiurids from the Dogger, some of 

 which he describes as new. The five primary plates immediately surrounding the 

 dorsocentral are invariably radial in position. The bursal clefts are sometimes 

 not visible in the fossil state, being occupied by the genital scales which have 

 been forced outwards from within by post-mortem contractions of the disk. An- 

 other result of these is the depression of its interbrachial regions, giving it a 

 rosette-like appearance. Fossil Ophiurids with regenerated arms are not un- 

 common. 



Fewkes ( ! ) finds that the genital bursae of the Q Amphiura squamata often 

 contain parasitic Copepods, together with packets of their ova; while the ovaries 

 of the host degenerate into an amorphous mass without traces of ova. The de- 

 velopment of A. -larvae thus being prevented, those of the G'opepod can mature 

 freely. 



V. Echiuoidea. 



See also Bury, supra, p 4, Cuenot, supra, p 6, Fewkes ( 2 , 3 ), supra, p 7, 

 Hamann, supra, p4, Korschelt('), supra, p3, Kowalevsky, supra, p5, Semon (*), 



supra, p 5, and Sluiter, supra, p 8. 



Pouchet & Chabry( 1 , 2 ) find that, by replacing the lime of sea- water with potash or 

 soda, the development of Urchin larvae is much retarded. When the proportion 

 of lime is reduced to '/ 10 , the larvae are normal for the first 40 hours, but are still 

 gastrulae at 60 hours. By 90 hours, however, they pass into a Pluteus stage, 

 with fore-, mid- and hind-gut, but are spherical in form and without arms, dying 

 after a few days. With more of the lime removed, few of the larvae reach the 

 gastrula stage, and none pass through it. When less lime is removed, the deve- 

 lopment of spicules is retarded, and the two frontal arms are replaced by a median 

 proboscis-like appendage. 



Boveri ( l , 2 ) has made use of the ova of Echinoids to study the phenomena of 

 fertilisation and the influence of the spermatozoon on segmentation. For an 

 account of his observations see Bericht for 1888 Allg. Entwick. p 11. From- 

 mann has studied the vital processes in the ripe unfertilised and fertilised ova of 

 Strongylocentrotus lividus, and concludes that both in its structure and in its vital 

 changes the yolk mass completely corresponds to the protoplasm of other cells. 

 For further details respecting its granular structure and radiate figures see infra, 

 Allg. Biologie. 



Duncan ( 2 ) has published a revision of the genera and great groups of the 

 Echinoidea to which he has added a detailed explanation of the current anatomical 

 nomenclature. 



Loven figures the perignathic girdle of Discoidea cylmdrica as a slightly 

 funnel-shaped ring composed of five broad pieces formed by the interradially 

 connected auricles. These are supported by the lateral parts of the ambulacra 

 which have no longitudinal ridge as in Pygaster, but only a transverse swelling 

 formed by a short succession of elevated ambulacral plates. In each interradial 



Zool. Jahresberielit. 1889. Echiiioderina. f 



