Angular Sea-Cucumher. 157 



For a good figure of the pseud-haemal system, and the commis- 

 sural junction between its ventral factors upon the first and 

 second segments of the intestine, see Sars, Oversigt af Norges 

 Echinodermer, 1861, Tab. xv. fig. i m, I. 



For the homology of the ' auriculae' of the Echinoidea, of the 



of the rows of feet which it supplies, and to be more or less physiologically similar to 

 the allocation of a series of rjaiujUons de rmforcemciit (for which see p. 132, mpra, 

 ibique citafa\ to the line of insertion of the feet as seen in Nereis regia. An exactly 

 similar separation of the nerve-cord into its two component halves, is to be seen in 

 many Serpulaceae ; see Quatrefages, Hist. Nat. Annelds, pi. iii., fig. 7. 8 ; Ann. 

 Sci. Nat., Ser. iii., torn, x., pi. 2, torn, xiv., pi. 10 ; and when we couple these facts 

 of comparative anatomy with the fact, that whenever the development of the nerve 

 centres of Invertebrata has been observed, that is to say, in Mollusca, Arthropoda, 

 and Vermes, these centres have been observed to be differentiated at a late stage 

 in the series of developmental changes (see page 109 sujira, and Claparede, Beobach- 

 tungen iiber die Anatomic und Entwickelungsgeschichte wirbelloser Thiers, 1863, 

 p. 87), it will appear probable that the physiological necessity which the akeady 

 radiate Echinoderm has for a radiate nerve-system, may be the regulating condition 

 of its peculiar arrangement, and that they are therefore, pro tanto, and as regards 

 their nerve system, approximated rather to the highest Annelids than to the Ne- 

 matoids, as suggested by Dr. Charlton Bastian. A timer homology for the trifid 

 nerve-system of the Nematoids, may be found in the nerve-cord of the singular 

 Annelid, Sphaerodorum peripatui, as figured by Claparede, I. c, pp. b°-B3, V^- xi., 

 fig. 17, where the super-addition to an organism, in other respects closely akin to 

 that of aNematoid, of motor organs, ia the shape of numerous pairs of comparatively 

 simple setigerous uniramous feet, has been accompanied by a corresponding addition 

 of trifid nerve-ganglia, which are united by commissures into a continuous cord. 

 Coming finally to the indications of affinity which the history of the various stages 

 of development is properiy held to point to, we may say that what is known of the 

 development of the Polychaetous and especially of the Tubicolar or Capitibranchiate 

 Polychaetous Annelids, shows that a closer relationship subsists between them and the 

 Echinodermata than between even the Gephyrean Vermes and these latter animals. 

 To judge of this similarity, it may be well to compare such figures as are given of 

 the development of Antedo7i Rosaceus {Comatula Rosacea) by Professor Wyville 

 Tliomson, Phil. Trans., 1865, pi. xxiv., xxv., and xxvi. ; of Synapia dhjitata by 

 Baur, Nova Acta, 1864, pi. iv. ; or such figures as are given of various forms of 

 Echinoderm larvae, from the memoirs of Miiller and others, in Bronn's Klassen und 

 Ordnungen des Thier-reichs, Taf xxxv., xxxvi., xxxvii., and especially Taf. xlvi., 

 with the fi-^ures given by ClaparMe, I. c. ; of the development of Leucodore, Spio, 

 Terebella and Magelona, Taf. vii., viii., ix., x., xi., and those given by Kefcrstein 

 and Ehlers, in their Zoologische Beitriige, Taf. viii. of Sipunculus. See especially 

 Claparfede's remarks as to the Tubicolar Annelids going in their early stages through 

 more typical forms than those they ultimately rest in, and as presenting us therefore 

 with instances of retrogressive metamorphosis, a possibility which is not rarely lost 

 sight of in such discussions as these. 



