194 



TURBELLARIA. 



FIG. 89. PLANA- 

 RIAN LARVA (PRO- 

 BABLY PLANARIA 

 ANGULATA). (From 

 Agassiz.) 



-possibly Plan, angulata, 



described by 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 stomodaeum. 



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 ventrally-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 incon- 

 spicuous prominences of the side of the body. Gotte 

 believes that the larva undergoes no further metamor- 

 phosis. 



A type of Planarian larva (figs. 89 and 90) 

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

 different from any other so far described, and 

 is remarkable for being divided into a series 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 



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 stoma- 

 doeum 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.) 



