XVI ASTERIAS PERIHAEMAL AND VASCULAR SPACES 449 



history in the larva appears to be a rudimentary counterpart of 

 the water-vascular system, since this organ in correspondence 

 with the general bilateral symmetry of the larva is at first 

 paired. Into this a special process of the genital stolon projects. 



(4) The "aboral sinus" (Fig. 192, ab), a tube embedded in 

 the dorsal body-wall running horizontally round the disc. The 

 aboral sinus surrounds the genital rachis (see p. 452) and gives 

 off into each arm two branches, the ends of which swell so as 

 to surround the genital organs. It has no connexion with the 

 axial sinus though the contrary has often been stated by Ludwig.^ 



(5) The " peribranchial spaces," circular spaces which sur- 

 round the basal parts of the papulae (Fig. 1^2, p. hr). 



Besides these, large irregular spaces have been described as 

 existing in the body-wall by Hamann " and other authors, but 

 for various reasons and especially because they possess no definite 

 wall they appear to be nothing more than rents caused by the 

 escape of CO2 gas during the process of decalcifying, to which 

 the tissues of the Starfish must be subjected before it is easy to 

 cut sections of them. 



• The question as to whether or not there is a blood system in 

 the Starfish has an interesting history. It must be remembered 

 that the examination of the structure of Echinodermata was first 

 undertaken by human anatomists, who approached the subject 

 imbued with the idea that representatives of all the systems of 

 organs found in the human subject would be found in the lower 

 animals also. So the perihaemal canals were originally described 

 as blood-vessels. Later, Ludwig ^ discovered a strand of strongly 

 staining material running in each septum which separates the 

 two perihaemal canals of the arm. Each of these radial strands 

 could be traced into connexion with a circular strand interposed 

 between the outer and the inner perihaemal ring-canals. This 

 circular strand again came into connexion with a brown, lobed 

 organ, lying in the wall of the axial sinus, and this in turn 



' Bronn's Thier-Reich, Bd. ii. Abt. 3, Buch ii. Seesternc, p. 617. 



2 BeitriUje zur Histologie der Echinodermen , Jena, 1889. Such sfjaces are always 

 to be seen in Asterina gibbosa when preserved with corrosive sublimate or other 

 acid reagents, but are absent when it is preserved with osmic acid and Mueller's 

 fluid. Though corrosive sublimate is usually regarded as a neutral salt, its aqueous 

 solution decomposes with tlie production of a certain amount of free hydrochloric 

 acid. 



' "Beitriige zur Anatomic der Asteriden," Zeitschr. wiss. Zool. xxx. 1877, pp. 

 122 ct seq. 



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