160 



TURBELLARIA. 



FIG. 89. PLANA- 

 KIAN LAEVA (PROBA- 

 BLY PLANARIA ANGU- 

 LATA). (From Agas- 

 siz.) 



Leptoplaua, as described hy Keferstein. Four large hypoblast cells 

 become surrounded by small epiblast cells, which commence to be formed 

 on the dorsal side. The hypoblast cells divide and arrange themselves in 

 two bilaterally-symmetrical rows. A small blastopore is left by the small 

 cells on the ventral surface, which communicates with an otherwise closed 

 and ciliated cavity which is formed between the two rows of hypoblast cells. 

 The blastopore would seem to remain permanently open, and to be placed 

 at the base of a deep pit, lined by epiblast cells, which constitutes the 

 stomodteum. 



The embryo now becomes dorsally convex, while the ventral surface 

 becomes marked with a median furrow and grows out laterally into 

 two lobes, and anteriorly into a ventral ly-directed upper lip. The whole 

 surface becomes ciliated, and the cilia are especially prominent on the 

 ventral processes and the summit of the dorsal dome. 

 A bunch of strong cilia becomes formed in front of the 

 dome, and a less marked bunch behind. The larva is 

 now stated by Gotte closely to resemble a Pilidium. 

 It soon, however, extends itself, and the two bunches 

 of cilia become situated at the anterior and posterior 

 extremities of the body. The ventral processes become 

 inconspicuous prominences of the side of the body. 

 Gotte believes that the larva undergoes no further 

 metamorphosis. 



A type of Planariau larva (figs. 89 and 90) possibly Plan, angulata, 



observed by Alex. Agassiz (No. 181), is very 

 different from any other so far described, and 

 is remarkable for being divided into a sei'ies of 

 segments corresponding in number with the 

 diverticula of the digestive cavity. In the 

 youngest specimen (fig. 89) the body was nearly 

 cylindrical, and divided into eleven rings, cor- 

 responding with as many digestive diverticula. 

 Two eye-spots were present. In a later stage 

 (fig. 90) the body was considerably flattened 

 and had approached more to the planarian form. 



the German abstract is too obscure to be placed iu the text, but the following are 

 the more important points which can be gleaned from it. 



The ovum becomes first divided into eight segments. By further division along 

 the equatorial zone, a ring of small cells is formed which becomes the epiblast. The 

 two poles are at this time formed of large cells. At one pole four small cells appear, 

 which are compared by Metschnikoff to the pole cells of the Diptera (vide Chapter 

 on the development of the Insecta). At the opposite pole a blastopore is formed 

 leading into a small segmentation cavity. The epiblast also now gradually grows over 

 the large cells. At the blastopore pole the large cells give rise to the hypoblast and 

 the small cells at the opposite pole assist in forming the epiblast. The blastopore 

 disappears, and with it the segmentation cavity, while the hypoblast, forming a solid 

 mass, becomes divided into two halves (Cf. Planaria Neapolitana.) The embryo be- 

 comes ciliated and begins to rotate; and the eyes, and somewhat later (?) the nervous 

 ganglion make their appearance. 



In the interior a wide cavity develops between the hypoblast cells, which becomes 

 ciliated and is placed in communication with the exterior by an invaginated stomo- 

 dffiuin which forms the pharynx. 



The larva now, as in Planaria Neapolitana, takes on a Pilidium-like form. Lateral 

 lobes and an anterior lip grow out from the under surface, and become covered 

 with long cilia, while at the upper pole a long flagellum makes its appearance. 



FIG. 90. PLANARIAN LARVA 

 (PROBABLY PLANARIA ANGU- 

 LATA). (From Agassiz.) 



