8 Ech. 
ECHINODEKMATA. 
he has not recognized the sanguiniferous or vascular nature of the tube 
surrounding the nerve-trunks and their branches, and he describes the 
central nerve-riiig as enclosed in a similar tube, on which Teuscher is 
silent. He further observed that the nerves for the suckers make their 
exit through the pore nearest to the axis of the ambulacrum, of the two 
pores belonging to each foot. The nerve-ring is composed of two con- 
centric portions, the separation of which is, however, only indicated by 
a furrow ; from the external, the ambulacral nerve-trunks are given off ; 
from the internal, ten branches to the digestive tube. Each ambulacral 
nerve-trunk terminates abactinally below the skin, which closes the pore 
of the ‘ ocular^ plate ; of eyes there is no vestige, nor was Frederiq capable 
of discovering the unpaired tentacle described by Agassiz. The cutting 
through of the nerve-trunks at their origin from the ring does not disturb 
the functions of the ambulacra, but disturbs their co-operation ; after 
the operation, the sea-urchin is unable to move from the place, or to 
change its position if laid on the back. Von Ankum (1) has examined a 
large number of male and female JE. esculentus^ and ascertained that in 
the majority the ovaria (or testes) are coalesced, always excepting how- 
ever the two on both sides of the rectum ; in the few specimens exa- 
mined of other genera {Strongylocentrus, Eeterocentvus, Tripneusles^ 
Echinometra). the same rule was found to prevail. He further describes 
the buckle- or S-shaped calcareous spicula of the sexual organs in E. 
esculentus ; in the genital glands and intestinal walls of Echinometra 
lucunter they have a great resemblance to those of the Calcispongia-.^ 
being typically tri-radiate (though with many individual variations) 
with a central canal and central protoplasmic filament, &c. 
Anatomy of Asteridce. Of the nervous and vascular systems T busciieu 
(/. c.) gives the following description, based chiefly upon Astropecten 
aurantiacus. The ring-shaped central portions of the sanguiniferous, 
aquiferous, and nervous systems are placed close together, encircling the 
mouth, above the calcareous ossicula forming the mouth-angles. The 
band-shaped nerve-trunks form the bottom of the ambulacral furrows, 
being the parts placed most externally ; inside of them are placed the 
chambered neural vessels ; and innermost, the aquiferous tubes. These 
bands have been falsely interpreted as consisting exclusively of nerve- 
substance, or as belonging chiefly to the skin ; the truth is that both 
elements are present, the dermal layer externally, commonly very 
intimately connected, the true nervous layer presenting the same 
structure as in other Echinoderms, especially in those (Crinoids, Holo- 
thurians) in which it is unprotected by a calcareous shell. It is con- 
tinued upon the inside of the basal portion of the suckers, where 
it ends abruptly ; it does not form the whole outer covering of these 
organs. It is also continued, with its dermal covering, into the un- 
paired feeler at the extremity of the arms ; the eye also is a specially 
developed part of the nervous band, crystal corpuscles being imbedded 
in its dermal layer. The principal neural vessel, immediately inside 
the nervous band, is subdivided by transverse (hitherto overlooked) septa, 
corresponding to the “ vertobrte*’ of the skeleton, in chambers, without 
any immediate mutual communication ; these chambers are further 
