REPORT ON THE OPHIUROIDEA. '269 



Astrocnida. 



Astrocnida, Lym., Ann. Soi. Nat., 1872. 



Disk divided into five radiating wedges by the pairs of radial shields, and, together 

 with the arms, covered by a minute pavement of flattened granules. The arms fork a 

 few times, but only near their ends. Teeth, tooth papillae, and mouth papilla} all similar 

 and spiniform. Side arm plates confined to under surface of arm and bearing several 

 rough, cylindrical tentacle scales (or arm spines). These plates are continued upward by 

 a double row of lumpy nodules homologous with upper arm plates, which bear minute 

 hooks, and which are continued over the roof of the disk, as irregular, concentric circles 

 of short, blunt spikes, or large granules bearing hooks. Two small genital openings in 

 each interbrachial space at the outer corners. 



A specimen of the rare Astrocnida isidis from the " Blake " dredgings afforded 

 a chance to examine a branching star, like Astrogomphus in outward appearance, but 

 resembling Triehaster in its few and widely- spaced arm forks. On making a vertical 

 section through the disk (PL XLVI. fig. 2), a curious and quasi-intermediate structure 

 is exposed. The digestive cavity recalls Gorgonocephalus in that it is more or less 

 pleated and pouched {St'), and is firmly attached to the roof of the disk wall ; but it is 

 Ophiuroid in being entirely free below, and partly so on its sides, having no radiating 

 lines of attachment, either along the arms, or in the interbrachial spaces. The only 

 vestige of such attachments is a stout septum, such as is found in Ophiurans lying 

 outside the wall of the stomach sphincter {du), and thus forming a closed ring tube 

 (inner perihaernal canal). It may more properly be called an adhesion of the floor of the 

 digestive cavity to the wall of the mouth where they are doubled over each other. 

 Between the upper side of the digestive cavity and the disk wall, and on top, and 

 on either side of each arm, lie the ovaries {S), which consist of almost separated ovoid 

 egg clusters, rather more than 1 mm. in length, containing round eggs about 2 mm. in 

 diameter. They are not connected with, or surrounded by any bursa, but lie directly 

 in the body cavity, into which penetrate the genital openings. The genital organs are 

 therefore strictly of an Astrophyton type, and discharge their products into the body 

 cavity, which is continuous and uninterrupted by radiating partitions. 



Astrocnida, and behind it Astrogomphtis, is nearest in relationship to the true 

 Astrophytons. Not only does the arm covering, with its double rings of minute hooks 

 shadow forth an affinity, but the internal structure, with a pouched digestive cavity and 

 ovaries lying free in the general body cavity is similar ; while the want of adhesions on the 

 under side of the digestive cavity and the closed ring tube about the mouth remind us of 

 the Ophiurans. But in reaching after some form which may bridge the way to these last, 

 we find, as generally happens in the animal kingdom, no piece that will fit. Ophiocreas, 

 which is properly a simple armed Astrophyton, is not intermediate. It is a synthetic form. 

 It has the teeth of Ewryale, the pleated digestive cavity suggestive of Gorgonocephalus, 

 the genital bursa and ovarial tubes similar to, yet not the same as, those of Ophiurans in 



(ZOOL. CHALL. EXP. PABT XIV. — 1S82.) 35 



