A. E. Verrill — Marine Nemerteans of JVe^o England, etc. 433 



holie specimens — a difficulty tliat holds with quite as nuu!h force in 

 many other groups of animals, which lose many of their characters 

 by preservation in any known medium. 



However, it seems to me that some of the large species referred to 

 Micrura by authors really belong to Cei'ebrattdus, especially those 

 like 31. fusca Mcintosh {non Fabr. sp.) 



The differences noted by Mcintosh in the muscular layers of the 

 proboscis appears to me of less importance than the special muscular 

 structure of the body-wall which enables the species of Cerehratalus 

 to swim actively at the surface, while the more slender and rounded 

 species belonging to Micrura (restr.) and Linens, so far as I have 

 observed, are unable to swim, and do not voluntarily leave the 

 bottom. 



The broad, flattened form of the body with thin tnargins is the 

 external expression of the internal musculature, adapting it to the un- 

 dulatory swimming motion. 



Cerebratulus lacteus (Leidy) Veniii. 



Meckelia frag ills Girard, Nord Arner. Monatsb., 1851 {non Goodsir, sp.) 



Meckelia lactea Leidy, Proceedings Academy Natural Sciences of Pliiladelphia, vol. 



V, p. 243, 1851, (young); Verrill, Invertebrate Animals of Vineyard Sound, p. 



336 [630], 1873, (young), nou 0. lacteus Hubrecht, Mont. s\). = Liueus lacteus 



Mcintosh. 

 f Meckelia Lizzice, Girard, Proc. Acad. Nat. Sci. Philad., vol. vi, p. 366, 1854. 

 Mecktlia iwjens Leidy, Marine Invertebrate Fauna of Rhode Island and New Jersey, 



p. 11 (143), 1*855; Verrill, Invertebrate Animals of Vineyard Sound, p. 336 [630], 



Plate XI.V, figures 96, 96a. 



PLATE XXXV, FIGURES 1, \a ; PLATE XXXVI, FIGURE 2 ; PLATE 

 XXXVII, FIGURES 1, Irt, \b ; PLATE XXXIX, FfGURES 19, 20, 21. 



Body flat, large and very long when full grown, sometimes becom- 

 ing fifteen to twenty feet long and upwards of an inch in breadth, 

 very contractile and changeable in length, breadth, and form. 

 While swimming the body is turned up edgewise and thrown into 

 many undulations and the motion resembles that of an eel, but is 

 less rapid. Tlie anterior part of the body for some distance back of 

 the head is, in usual extension, narrower and thicker than the rest, 

 with the margins rounded ; the body then expands rather rapidly 

 in breadth and at the same time becomes more flattened while the 

 margins become thin and pale, and throughout the rest of its length 

 the body continues thin and flat, gradually decreasing in breadth 

 and thickness toward the ])osterior end, which is usually obtuse, or 

 slightly emarginate, but occasionally, or when perfect, terminates in 



