xv ECHIUROIDEA ANATOMY 437 



passes from the ventral vessel and encircles the intestine, opening 

 into the posterior end of the dorsal vessel. In Ecliiurus the 

 same vessel encircles a stout muscle which runs from the base of 

 one of the ventral bristles to the other. In Tkalassema Lankester 

 states that the fluid within the vessels is colourless, and does 

 not contain corpuscles similar to those in the body-cavity fluid. 



The " brown tubes " or nephridia vary in number in the 

 Echiurids. In the female Bonellia there is but one ; in B. viridis 

 the right, in B. minor the left usually persists. In shape, 

 colour, contractility, and minute structure they closely resemble 

 those of Sipunculus. Hamingia, is said to have a pair of brown 

 tubes; Echiurus has two pairs, except E. chilensis, which has 

 three ; their internal openings are produced into long coiled slits 

 in some genera. Tkalassema gigas has one pair ; Th. neptuni, 

 Th. baronii, Th. formosulum, and Th. exilii, two ; whilst Th. 

 vegrande, Th. moebii, Th. erythrogrammon, Th. caudex, and Th. 

 sorbillans have three pairs. 



The nervous system consists of a ventral cord lying in the 

 body-cavity, as in the Sipunculoidea, but attached to the skin, 

 and of a circumoesophageal ring. With the growth of the pro- 

 boscis this ring is drawn out, and the two branches run along 

 the sides of the proboscis and unite at the tip. There is no 

 specialisation of brain, nor are any special sense organs present, 

 but the ventral cord gives off paired nerves at regular intervals, 

 which, uniting dorsally, form rings in the skin in some and prob- 

 ably in all species. 



The perivisceral fluid is of a dark brown colour in Thalasscma, 

 containing numerous spherical corpuscles deeply impregnated, 

 according to Lankester, with haemoglobin, and also containing 

 granules of a brown pigment. Haemoglobin is also found in 

 certain of the muscles and in part of the epithelial lining of the 

 body-cavity. Lankester also describes the presence of haemo- 

 globin in the corpuscles of the perivisceral fluid in Hamingia. 



The genital glands are, like those of the Sipunculoidea, formed 

 by a special development of the cells lining the body-cavity. 

 These cells are massed together along the wall of the ventral 

 blood-vessel. In Echiurus and in Thalasscma the cells break off 

 and float in the body-cavity, developing into ova and spermatozoa. 

 In Bonellia each cell does not become an egg, but a mass of cells 

 breaks oif, one of which increases in size at the expense of the 



