HOMOLOGIES. 149 



previous form. The broad, fleshy collar has two powerful conical bands of fibres (continuous 

 with the dorsal belts) attached to the chitinous process on the dorsum, while ventrally a special 

 bundle of fibres passes from the collar to the anterior margin of the trunk. 



The dermal layers of the body agree in both species, as also does the external (longitudinal) 

 muscular coat; but though certain circular muscular fibres lie under the latter, they are so 

 indistinct as scarcely to merit the name of a special layer. 



A considerable difference is apparent on opening the body-cavity, as at first sight the 

 branchial arrangement characteristic of the former examples seems to be absent. On careful 

 inspection, however, many minute, transparent, chitinous processes are found in the somewhat 

 thickened membrane behind the dorsal valves. These processes have the form of a pointed 

 molar tooth with very long fangs, and a fissure passing up the centre of the tooth to the crown. 

 Some of the fangs or processes are bifid at the tip, each division diverging with a curve from the 

 main stem. In all probability they form a short double row, after the type of the former species ; 

 but the specimen is not in a condition to bear searching investigation. 



The digestive canal and accessory glands have a similar character to those in the foregoing 

 species ; the former being distended with muddy sand containing many Foraminifera and other 

 microscopic organisms, the latter chiefly grouped along the dorsal area. 



This species would not seem to approach any yet described. 



In reviewing the several features presented by these curious forms, and contrasting them 

 with what is known of Nemertean anatomy and physiology, the following reflections occur : — 



In both the cutaneous tissues have the closest similitude as regards ciliation and minute 

 structure. In Balanoglossus, however, the basement-membrane underneath the latter is more 

 differentiated, and assumes a slightly fibrous appearance. 



In the arrangement and histology of the muscles of the body-wall they much resemble each 

 other. 



The digestive system is similar. Both have a ciliated chamber divided into two great 

 regions, represented by the first or branchial, and the succeeding division in Balanoglossus, and 

 by the oesophageal and alimentary cavity proper in the Nemertean. The peculiar rete mirabile 

 over the oesophageal region of the Nemertean, and the elaborate branchial circulation of Balano- 

 glossus are apparently homologous. The minute structure of the proper wall of the chamber is 

 closely allied. Moreover, while the " liver "-tissue is separated into elongated sacs in Balano- 

 glossus, and simply diffused over the alimentary region in the Nemertean, its histological features 

 are nearly identical. The mouth and anus are also similarly arranged. 



With regard to the " proboscis " of Balanoglossus and that of the Nemertean I fear there is 

 no homology \ indeed, I would be inclined to regard the anterior region in Balanoglossus rather 

 as the homologue of the Nemertean snout. The pore at the tip in the former would therefore 

 correspond with the aperture for the proboscis in the latter, the mouth in both being placed a 

 considerable distance backwards. 



A great divergence happens in regard to the nervous system. It is not yet sufficiently 

 understood to enable us to form a correct idea of its relations in Balanoglossus, while it is con- 

 spicuous in the Nemertean. 



There is considerable similitude in the circulatory system. Both have a main dorsal and two 



