TETRASTEMMA DORSALIS. 173 



end of the body to the other, but they do not form a continuous stripe. In some there is also 

 a distinct brown lateral line. The other variety is reddish -brown, with a pale yellow dorsal 

 stripe from snout to tail. The sepia-brown grains of the dorsum are placed on a reddish- 

 brown ground. As soon as the pigment of the body becomes dark enough, a fine purplish lustre 

 is produced by the play of light on the cilia, and the animal assumes a rich velvety aspect. In 

 these dark examples, with a reddish-yellow central stripe, the under surface of the snout generally 

 presents two pale symmetrical ovoid spaces a little behind the line indicating the mouth. In 

 some cases the yellowish grains are scattered over both dorsal and ventral surfaces, and are quite 

 characteristic of the species. In a large example dredged in Bressay Sound the dorsum was curi- 

 ously variegated with patches of cinnamon-brown on a general ground-colour of pale brown speckled 

 with yellowish grains, the pale brown of the ventral surface having a few specks of the same hue 

 as the dorsum. The ventral surface is somewhat pale in all cases, and occasionally assumes a 

 dull brownish-orange posteriorly. When floating with the ventral surface uppermost, a brownish 

 margin is generally visible. Young examples are occasionally reddish-orange. 



Head somewhat truncate and even notched in front, slightly narrowed posteriorly towards 

 the cephalic furrows, and very little flattened. Eyes four, nearly in a square, the first pair being 

 further from the tip of the snout than the last are from the cephalic furrows. They are deeply 

 situated, and somewhat lateral in position — from the roundness of the snout when seen from 

 above, and in profile appear considerably below the dorsal line. A variety from the harbour 

 of Symbister, in Whalsay (Shetland), has the head somewhat pointed and better defined than 

 usual, and the tail is also more tapered. 



Cephalic furrows. — The anterior furrows are not visible on the dorsum. The posterior slant 

 inwards and slightly backwards some distance behind the posterior eyes, meeting in the middle 

 line. On the ventral surface they are directed slightly forward. 



T. dorsalis is a marine rather than a littoral form, and sometimes occurs in vast numbers 

 clinging to the debris of various seaweeds dredged in the laminarian region. It is hardy in con- 

 finement, either gliding with scarcely an undulation of its body, or rolling its snout from 

 side to side in a peculiar manner as it progresses along the glass. It is also fond of 

 enclosing itself in a tough transparent sheath, which is fixed to the wall of the vessel at 

 the water-line. The sheath is highly elastic, and, while remaining perfectly transparent under 

 a high power, is yet minutely streaked with translucent granules or specks, which are not 

 due to fine creases or folds. The animal often reverses itself in the tube, and is sometimes 

 doubled therein. It is curious to watch the pertinacity with which it progresses in a definite 

 direction to stretch itself along the water-line. 



The ova are deposited in the beginning of September, but some latitude is necessary in this 

 respect, since specimens from deep water produced ova in June. 



This species was brought into notice by P. C. Abildgaard in the fourth volume of 0. F. 

 Mutter's celebrated * Zoologica Danica ;' and though he did not observe the eyes, his account is 

 otherwise good. He terms it an eyeless brown ' Planaria/ with a pale ventral surface and a 

 white dorsal line, and which constantly twists itself and loves to swim on its back. M. de 

 Quatrefages, again, made a distinction between two varieties of this species, one of which 

 {(Erstedia tubicola), he said, formed a tube, while the other (0. maculata) did not. Moreover, he 

 elevated them into a new genus— characterized by the sublateral position of the nerve-trunks, 

 and their cylindrical bodies. Their structure, however, is in all respects strictly conformable to 



