OF PLANOCERA ELLIPTICA. 321 



the back and bellj. Indeed, wdien speaking 0/ back, belly and sides, it is in refer- 

 ence to the eye specks, which we consider as appertaining to the anterior region, for, 

 in their motion, the larvte keep in no permanent position or orientation. Almost any of 

 the regions may be directed forwards; it is a perpetual rolling rather than a head- 

 wards progression ; no region yet predominates exclusively over the other regions. 



Towards the eve of the larval life, the larvse are no longer moving in the open medium. 

 They keep nearer the bottom of the vessel, move slower, but are still subjected to those 

 contractions of their body, which alternately and temporarily modify their shape. 



XIII. THE CHRYSALIS. 



That larva which was seen so lively, so active, so plastic, so polymorphic, becomes 

 a mummy-like body, an immoveable chrysalis (fig. 104), eight or ten days after it first 

 began to revolve as an embryo (fig. 52, 53 and 55). 



This chrysalis is an elongated, slightly curved body, semicylindrical in sliape, con- 

 cave above and flat below, as shown by a transverse section (fig. 105). Opaque at 

 both extremities, whilst the middle region remains transparent, no organs are discerni- 

 lile in any of the regions. The external envelope, or case, is a tough membrane, 

 through which the microscope can hardly penetrate. When crushed between two 

 glasses, the cellular contents then escape, leaving the case empty. 



I am not prepared to give any information in relation to the length of time which 

 the animal remains in this state, having lost a great number of these little bodies 

 wdiich could scarcely be distinguished amongst the sand and organic substances 

 always floating in the water; and when the latter had to be changed, many of them 

 were unavoidably lost. On this account my investigations, have come to a tempo- 

 rary close at this point of their history. 



For a time, I was still in doubt as to whether these singular chrysalis-like bodies 

 were really a normal state of existence in the embryogeny of Planarians, or an ab- 

 normal one, in consequence of the limited space in which the larvae had been kept. 

 But soon afterwards, having had an opportunity to observe the same bodies out at sea, 

 in their natural element, there could be no hesitation in regarding this chrysalis state 

 as a natural period in the genetic development oi Planocera elliptica. 



How long this period will last, is a question now to be investigated; also, whether 

 the perfect animal is the next step, or whether there are other stages of development, 

 or other metamorphoses. 



XIV. CHRONOLOGICAL SKETCH. 



In 1849, a chronological narrative of the development of P/a;iOcera e/ZZ/j/iVa was read 

 before the American Association for the Advancement of Science,* and subsequently 

 before the Society of Natural Sciences of Neuchutel, Switzerland. f 



♦■Proc. Amor. Assoc. Adv. Sc. second meeting, held at Cambridge, in August 1840. Cambridge 18-50, 39^. 

 tBuIl. de la Soc. des Sc. Nat. de Neuch. II. 18-30, 300. 



81 



