METAZOA VERMES. 319 



are remarkable for complicated processes of development 

 which illustrate in the majority of cases great abbrevia- 

 tion in the life history. 



The marine Turbellaria or Polyclads are mostly littoral 

 animals, but a few genera are pelagic. 



Alaurina composita Metsch. (PL 788), was found at the 

 surface of the sea. Its body is" segmented 1 and is long, 

 narrow, and covered with cilia. It has a tactile organ, 

 the proboscis, at the anterior end. There are stiff hairs 

 at the posterior end, and sometimes paired setae occur on 

 the sides. 2 



The life history of those Turbellaria which pass 

 through a metamorphosis begins (after the embryo is 

 free from the egg) with the stage known as Miiller's larva 

 (PL 789, fig. i, dorsal view; fig. 2, ventral view; prob- 

 ably figures of the genus Thysanozoon 3). The body is 

 somewhat elongated and possesses eight finger-like lobes 

 around which extends a band of cilia. Three of these 

 lobes are dorsal (fig. i, the posterior pair of lobes and 

 the single median lobe just back of the eyes), two lateral 

 (fig. i, the anterior pair), and three ventral (fig. 2, a short 

 pair and a single one just back of the eyes). The mouth 

 is in the center of these three lobes, and can be seen in 

 fig. 2 and also through the body in fig. i. When this 

 swimming larva changes into the creeping adult, the lobes 

 grow smaller till they disappear altogether, the ciliated 

 band is absorbed, and the form becomes flattened. 



The more complete development of a Turbellarian is 

 shown in PL 790, figs, i-io which represent Yungia 

 aurantiaca (figs. 1-8) and Thysanozoon brocchi (figs. 9, 



1 Metschnikoff, Ann. and Mag. Nat. Hist., (3) XVIII, 1866, p. 

 6 1 ; Huxley, in his Invertebrata, 1897, p. 157, says, "None are 

 divided into distinct segments except the genus Alaurina, in which 

 there are four." 



- Quart. Journ. Micr. Sci.. XXXIV, 1893, p. 449. 



3 Korschelt and Heider, Text-book of the Embryology of Inverte- 

 brates, part i, 1895, p. 1 66. 



