580 THE ANIMAL KINGDOM. 



toda, some Polyclad Turbellaria and Nematoda. An almost radial sym- 

 metry in the Turbellaria in question is noticeable, but the ventral position 

 of the trunks destroys an otherwise apparent resemblance to Ctenophora. 

 Complete circular nerve-rings connecting the longitudinal trunks from place 

 to place are present in some Gephyrea, Trematoda, and Nematoda. The 

 predominance of the dorsal nerve-cord and the great development of a 

 diffuse sub-epidermic nerve-layer in Enteropneusta, the varying positions 

 of the two lateral cords in Nemertea, i.e. lateral, more ventral or more 

 dorsal, and their occasional supra-anal union, are points bearing on the 

 supposed relationship of these groups to Chordata. Organs of special sense 

 occur in the shape of sense-cells with sense-hairs, aggregated into remark- 

 able lateral sense-organs in the Chaetopod family Capitellidae ; of sensory 

 papillae, of eyes and otocysts. But the last-named are far from common. 



A digestive tract is not even indicated in development in Cestoda and 

 Acantkocephala, probably in consequence of a long continued ancestral 

 endo-parasitic life dependent on a supply of ready digested food. It is 

 rudimentary in most male Rotifera, and in the sporocyst of Trematoda. 

 It is replaced by the central parenchyma of the body, into which food 

 passes through the mouth in the Acoela among Turbellaria. The form of 

 the tract varies. Sacculation more or less distinct occurs in segmented 

 Vermes. There is no anus in Trematoda or Turbellaria, and, what is more, 

 there is no evidence to show that it has become aborted. When present it 

 is terminal, ventral and sometimes dorsal (some Chaetopoda, Sipunculid 

 Gephyrea, Hirudinea). It is primitively dorsal in the larval Balanoglossus 

 Koivaleivskii (Enter opneustd}, but becomes terminal by the abortion of the 

 sub-anal portion of the body. The existence of laterally placed respiratory 

 apertures opening from without into the anterior part of the digestive tract 

 may, among other features, indicate a connection between the Enteropneusta 

 and the ancestral forms of the Chordata. 



A vascular system is developed in Enteropneusta, most Chaetopoda, the 

 Polygordiidae among Archi- Annelida and the Gephyrea, with the exception 

 of Priapulidae \ It consists of a system of tubes, some portion of which 

 is contractile, completely closed off from the coelome. In Hirudinea the 

 vessels appear to communicate with the remnants of the coelome as well 

 as with the secondarily formed metacoelome. Haemoglobin occurs in the 

 vascular plasma of many Chaetopoda, of Gnathobdellidae among Hirudinea, 

 in the blood corpuscles of some Nemertea. Another respiratory pigment, 

 chlorocruorin, is found in some Chaetopoda. Specialised haemoglobin- 

 tinted corpuscles are found in the coelome of a few Chaetopoda, and the 



1 The Nemertea also possess a vascular system. The cephalic vessels are spoken of by 

 Oudemans as lacunar, and Hubrecht regards the whole system as well as the cavity of the proboscis- 

 sheath as remnants of an archicoele. The communication in some Nemerteans between the 

 nephridia and the vascular lacunae is in favour of this view. 



