122 DEVELOPMENT OF THE ANOPLA. 



cut, fig. 7), and by-and-by it assumes a free existence. In the species described by Leuckart and 

 Pagenstecher from Heligoland, no caudal styliform process was observed ; but in J. Muller 

 and MetschnikofTs forms this was present, and such may in all probability be the young of 

 Micrura fasciolata (as mentioned by the former) or some other closely allied species. 



E. MetschnikofTs summary of the development of the Nemertean in the Pylidium is as 

 follows : — 



i . The commencement of the Nemertean body is in the form of two pairs of cutaneous 

 processes, which not only develop the body of the worm, but also the amnion. 



2. Two median vesicles are produced, which at a later period become connected with 

 lateral ducts. 



3. The four structures developed from the cutaneous processes, which represent the future 

 germ-fold, appear to be fashioned from two germ-leaves. Erom the outer leaf is formed the 

 epidermis and central nervous system, from the inner the muscular coat (and perhaps also the 

 circulatory system). 



4. Through the coalescence of these four processes, primitive folds representing the future 

 ventral surface, together with the head of the Nemertean, are developed; whilst the dorsal 

 coverings are formed subsequently. 



5. The proboscis is developed in the form of a simple process at the anterior part of the 

 germ -streaks. 



The reproductive organs of Noiospermus Jlaccidus are correctly represented by (Ersted, 

 but his drawing of the spermatozoa is inaccurate, since he shows a simple spindle-shaped 

 body without a filament. M. de Quatrefages observes that the reproductive organs are 

 digitate in Borlasia anglia, and figures them after this manner ; but such is scarcely a correct 

 definition ; neither have any cilia been detected in connection with these structures. Indeed, he 

 has probably mistaken the digestive canal and its sacculations for the reproductive system, as he 

 mentions that out of season the caeca are filled with a fluid more or less opaline. M. van 

 Beneden found the ovisacs to contain from one to a hundred ova in his Nemertes communis ; but 

 though deposited in a membranous sheath in September, no change had ensued in November. 

 His figure of the spermatozoa of this species is incomplete, as no tails are present, and he 

 describes them as simple rods. He makes the interesting statement, that in the same animal he 

 found the embryos in some ova covered with vibratile cilia even in the body of the parent, 

 while others were only fecundated during or after deposition. The young Nemertean described 

 by Dr. Busch, under the name Alar dm caudatus, would seem to have some relation to Micrura, 

 since it possesses a very distinct style at the posterior extremity. 



Although I am not quite free from doubts concerning the exact position of the curious larval 

 animal mentioned by Mr. Alex. Agassiz 1 as a further stage of the type first noted by the 

 distinguished zoologist of Stockholm, Prof. Loven, it may be well to conclude the present section 

 by a few observations thereon. In the early stages it is a somewhat club-shaped animal, 

 having a circlet of long cilia anteriorly, and another posteriorly in front of the anus. Behind the 

 anterior ring of cilia, the mouth opens into an oesophagus, followed by a stomach and intestine. 

 As the animal gets older two eyes and a pair of short cephalic tentacles appear, while the body 

 becomes much elongated and distinctly segmented. At a further stage a remarkable retrograde 

 metamorphosis ensues, whereby it loses the anterior and posterior ciliated rings, the tentacles, and 



1 ' Ann. Nat. Hist./ 3rd ser., xix, p. 208. 



