ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 55 



longispinus, which are placed round the arms, and which during life may- 

 be seen to be continually moving, their tips describing a circle. These 

 spines are from 1-3 mm. in length, according to the size of the animal. 

 On the surface there are a number of sensory prominences. Like the 

 other spines, these are attached to a hemispherical tubercle. Around 

 their base is a nerve-ring, whence fibres pass to the subjacent musculature 

 and to the tip of the spine. There is a rich muscular supply, which is 

 cylindrical in form, and is made up of transversely striated fibres. This 

 transverse striation is very rarely to be detected in specimens which have 

 been preserved in alcohol. 



The nervous system of a few Echinids was examined, and an elaborate 

 account is given. Nerve-fibres are to be found throughout the epidermis, 

 whence they pass into the cutis. At the middle of the paired ambulacral 

 plates are longitudinal canals. These begin at the apical pole beneath 

 the fine intergenital plates, and extend to the masticatory apparatus. 

 They are formed from the schizocoel, and lie in the layer of connective 

 tissue. Here, too, are the five radial nerve-trunks which, in the 

 Asteroidea, lie in the ectoderm. The trunks consist of very fine nerve- 

 fibres and ganglionic cells, together with a cellular investment, which is 

 partly formed of supporting cells. This epithelium may be regarded as 

 the homologue of the epithelium of the ambulacral grooves of star-fishes, 

 for it is not only the nervous mass, but also the whole epithelium that 

 has come to lie in the mesoderm, as in Holothurians. From the nerve- 

 ring branches are given off to the oesophagus, which extend over the 

 whole of the enteric tract. 



The blood-carrying spaces consist of fine longitudinal canals and a 

 circular space surrounding the nerve-ring. These structures in Echinids 

 have nothing to do with the true blood-lacunae, which arise from the 

 blood-lacuna-ring, which lies on the surface of the " lantern," as a 

 ventral and dorsal enteric lacuna. From the dorsal lacuna branches are 

 given off, which surround the glandular organ (or " heart " of earlier 

 authors). In its terminal portion the lacunae of the anal blood-lacuna- 

 ring are brought into connection with this organ. The anal lacuna 

 passes into a circular schizocoel-sinus, which surrounds the anus ; from 

 it blood-lacunae are given to the generative organs. 



Dr. Hamann describes a canal from the water-vascular ring as 

 passing into the " Polian vesicles " ; the canal opens into their cavity 

 while blood-fluid circulates in lacunae in the wall of connective tissue, 

 and these lacuna? are in direct connection with the blood-lacuna-ring. 



In the Spatangida the five longitudinal canals and an oesophageal 

 sinus communicating with them are present ; the true blood-lacuna- 

 ring has, however, disappeared with the lantern, and the dorsal and 

 ventral enteric lacunae open into the sinus. The dorsal lacuna runs 

 beside an enteric vessel, which arises from the circular canal that 

 surrounds the mouth. Later on, this water- vessel and the enteric lacuna 

 communicate with one another, and extend as far as the true stone- 

 canal. In this way a connection is effected between the water-vascular 

 and blood-lacuna-systems — or, in other words, between spaces of endo- 

 dermal and schizocoelic origin — such as has not been observed in any 

 other group of Echinoderms. We may well suppose that this arrange- 

 ment is secondary, since the Spatangida are palasontologically the 

 youngest form. 



The ovoid gland or so-called heart is a remarkable organ ; so far as 



