ECHINOIDEA 325 



2. Brisaster Gray. 



Test low, elongate. From the sides of the peripetalous fasciole 

 proceeds a lateral fasciole, running backwards, below the peri- 

 proct. Frontal ambulacrum very broad and deep. Three genital 



1 2 



Fig. 188. — Apical system of Heyniaster expergitus (l) and Brisaster fragilis (2). 

 (After Th. Mortensen, '' Ingolf Ech.,\\. (1) and Loven, Etudes sur 

 les Echinoidees (2j.) 



pores. Madreporite j^rolonged backwards so as to separate the 

 posterior genital and ocular plates (Fig. 188, 2), the apical 

 system being of the " ethmolytic " tjrpe. 



Only one species of this genus known from the British (and 

 N.E. Atlantic) seas. 



1. 5nsa5^er/rag'i7i5 (Diiben and Koren). (Fig. 189.) 

 (Syn. Brissus, Tripylus, Schizaster fragilis (Diib. and Kor.).) 



Test low, but rising gently towards the posterior end ; a deep 

 notch in the anterior end, on account of the deepened frontal 

 ambulacrum. Anterior paired petals about three times as long as 

 the posterior ones. Posterior interambulacrum usually raised so 

 as to form a fairly distinct keel on the upper side. Apical system 

 posterior to the middle of the test. No genital pore in the madre- 

 porite, thus only one genital pore in the right, two in the left side. 

 Usually the right, more rarely the left side of the test somewhat 

 projecting beyond the other. The lateral fasciole is rather often 

 indistinct, but the part of it below the anal area is usually quite 

 distinct. Globiferous pediceUariae usually rather conspicuous, 

 enclosed by a thick, often dark pigmented skin ; the valves 

 (Fig. 187, 2-3) are very characteristic in having a single end tooth, 

 at the side of which the opening of the poison gland is found. 

 Colour in life a dark bro^\Tiish, which usually keeps well in alcohol. 

 Reaches a size of up to 90 mm. length of test. 



It is a rather fragile form, which lives on a soft mud bottom ; 

 its intestine is filled with mud, the organic substances in the mud 



