538 INVEBTEBBATE MOBPHOLOGT. 



In connection with the stone-canal a peculiar body is 

 developed in most forms. Its function is a matter of ques- 

 tion, it having been at one time taken for the heart and at 

 another for a gland. It is generally termed the ovoid gland 

 (Fig. 265, og) and consists of a mass of cells, derived from the 

 peritoneal lining of the enterocoel, grouped together to form 

 a more or less solid mass. The oral end of the gland is pro- 

 longed into a cordlike structure which seems to enter into 

 close relationships with the oral lacunar ring (see below), while 

 at the other it is continued out to enter into close relationships 

 with the reproductive organs in a manner that will be de- 

 scribed when treating of those organs. Surrounding the 

 gland is a sinus — the axial sinus (Fig. 265, as)— separated off 

 from the enterocoel and, in some forms, in communication 

 with the ampulla of the stone-canal, and the portion of the 

 gland which passes off towards the reproductive gland is also 

 surrounded by a sinus, or rather lies in the wall of a sinus 

 which may or may not communicate with the axial sinus but 

 has, like it, origin from the general enterocoel. 



What has been termed a blood system is usually present, 

 consisting of a tubular ring surrounding the oesophagus, and 

 lying between the hydrocoel-ring and the nerve-ring. Five 

 branches may extend off from it along the radii, preserving 

 the same relations to adjacent structures as does the ring. 

 These spaces seem to be schizocoelic in their character, and 

 may be termed the schizocoslic ring and radial schizocoelic sinuses 

 in order to avoid confusion with another system of vessels 

 which sometimes lie within the sinuses and have also been 

 termed blood-vessels. This latter system may be termed the 

 lacunar system, and is composed of a network of vessels lying 

 in the walls of the intestine, and collecting usually into a 

 perioesophageal ring or plexus (Fig. 265, Ir), with which also 

 the ovoid gland comes into connection. In the Echinoids, 

 as has just been indicated, prolongations of this perioesophageal 

 ring or plexus extend ovit in the radial schizocoelic sinuses. 



The fluids contained in the sinuses, lacunae, hydrocoel and 

 enterocoel are all very similar, consisting of a plasma contain- 

 ing amoeboid cells sometimes deeply pigmented. In a few 

 forms haemoglobin is present ; in the Ophiuran Ophiactis it is 



