486 W. a. Coe — Anatomy of Cereoratulus lacteus. 



cular fibres comes in, which increases in strength farther back. A 

 short distance in front of the attachment of the proboscis this layer 

 greatly increases in thickness, forming a sort of sphincter (Plate XII, 

 fig. 2, rs) between the rhynchodaeum and the proboscis-cavity. Near 

 the posterior end of the rhynchodseum longitudinal muscular fibres 

 occur between and outside the circular muscles. These fibres 

 together with those from the proboscis are firmly attached to the 

 cephalic tissues (at x, Plate XII, figs. 1, 2) on all sides except where 

 they are interrupted by the blood-lacunae. Thoughout nearly the 

 whole length of the rhynchodgeum its walls are surrounded by the 

 fluid contained in the cephalic blood-lacunae [cl) and it is not improb- 

 able that the sea-water contained in the rhynchodaeum, as well as 

 that in the oesophagus, may aid in respiration. 



The proho8ci8-sheath (Plate X, figs. 3, 4 ; Plate XI, figs. 2, 3, 4 ; 

 Plate XII, figs. 1, 2, 7, jos) forms a tube with muscular walls reach- 

 ing from the origin of the proboscis in front of the brain nearly to 

 the 230sterior end of the body. The proboscis usually lies coiled up 

 in the anterior portion of the cavity of the proboscis-sheath and is 

 surrounded by a corpusculated fluid bearing a close resemblance to 

 blood. The anterior end of the proboscis (Plate XI, fig. 2 ; Plate 

 XII, figs. 1, 2, a?) is attached to the anterior end of this sheath and 

 to the cephalic musculature on all sides, com^^letely closing the cavity 

 of the proboscis-sheath, or rhynchocoelom. 



When the walls of the proboscis-sheath contract the rhynchoccelomic 

 fluid is forced against the attachment of the proboscis, thereby caus- 

 ing that organ to be protruded by eversion. Ordinarily the proboscis 

 is everted for a small portion of its length only. When the animal 

 is greatly irritated the muscular walls contract with such violence as 

 to rupture the attachment of the proboscis and expel it without 

 eversion. Most Nemerteans are said to withdraw their proboscides 

 by means of the retractor muscles. When, however, it is considered 

 that a large portion of the proboscis is withdrawn into the sheath 

 much farther than the attachment of the muscle to the wall of the 

 latter, and how very delicate this muscle is, it would seem that the 

 proboscis must be withdrawn by the action of the muscles of the 

 sheath aided, probablj'-, by those of the body- wall, rather than 

 by the contraction of the retractor muscles. This must necessarily 

 be the case in this species, in which, as stated below, the retractor is 

 entirely wanting. This muscle seems, rather, to function in limiting 

 the distance to which the proboscis can be extended. In the Enopla 

 it is just sufiicientl}'' long to allow the stylet to project at the 

 extremity of the everted proboscis. 



