and Anatomy of the Thalassema and Echiurus. 377 



roe of osseous fishes, and are about the size of millet -seed. 

 Examined under the microscope, the egg appears as a highly 

 transparent globule, enclosing towards its centre a number of 

 smaller globules or cells. 



The structure of Thalassema Neptuni is in all respects iden- 

 tical with Echiurus, the only difference, and that an unim- 

 portant one, being the less complicated arrangement of the 

 intestinal tube. 



The oral and anal hooks and spines of Echiurus, and the 

 oral hooks of the Thalassema, are protruded and withdrawn 

 exactly as the sette and booklets among the Annelides. 



From the anatomical description we have now given, it is 

 evident that the genera Echiurus and Thalassema must be 

 arranged in the class Echinodermata. The body filled with 

 sea- water — the respiratory apparatus — the digestive system — 

 and the intestinal venous trunk are the leading anatomical 

 peculiarities, and are characteristic of the Echinodermatous 

 animals. The colour and circulation of the blood, the want 

 of an aquiferous system, the ventral nervous cord, and the 

 muscular system, shew the relation of these animals to the An- 

 nelides, and prove that the transition from a vermiform radiate 

 animal to a true articulate animal, is effected by the symme- 

 trical atrophy and hypertrophy of certain^of the radiate ele- 

 ments in each riner. 



EXPLANATION OF PLATE VII. 



Fig. 1 . Echiurus vulgaris, of the natural size, as seen from the ventral 

 aspect. 



Fig. 2. Echiurus vulgaris laid open from the back, the integuments 

 stretched, and the greater part of the intestinal tube removed; a 

 the pharyngeal portion of the intestinal tube; b the oesophageal 

 portion ; c c the portion corresponding to tic stomach and in- 

 testine, 'which is of great length in this animal. The venous 

 trunk is seen coursing along its free edge, and branches of the 

 ventral vessel (artery) ramify on its attached or mesenteric edge ; 

 d rectum; e e respiratory sacs, covered with ciliated processes; 

 ffff generative sacs ; gg ventral vessel or arterial trunk ; h the 

 arterial branch which, surrounding the right genital hook, pro- 

 ceeds to the pharyngeal portion of the intestine, for the purpose of 

 supplying it with blood when the animal is boring ; i i i the pha- 

 ryngeal artery, and the two oral arterial circles; ltk the nervous 



