562 



COMPARATIVE ANATOMY 



from behind forward, in the latter, from before backward. A pulsating 

 heart vesicle, which serves to propel the blood, though not belonging 



to the vascular system, 

 is found in the proboscis, 

 above the intestinal 

 CcTcum. 



The sexes are sepa- 

 rate in the Entero- 

 pneusta. The gonads are 

 tubes or sacs which lie 

 in longitudinal rows in 

 the anterior region of 

 the trunk (in and behind 

 the posterior branchial 

 region) and open through 

 dorsal genital apertures. 

 There are no copulatory 

 organs. Reproduction 

 is sexual. Development 

 is with metamorphosis 

 (in which case the larva 

 ■^ is known as the Tornaria) 

 or with abbreviated 

 metamorphosis. 



Marine, inhabiting 

 sand or mud. Four 

 genera : Ptyehodera, 

 Glandieeps, Sehizoear- 

 dium, Balanog-lossus. 



I. Outer Organisation 



(Fig. 455, A, B). 



In the long worm-like 

 body, three principal 

 regions can be distin- 

 guislied, corresjionding 

 with three consecutive 

 sections of the cct'lom : 

 these are the probos- 

 eidal region, the collar 

 region, and the trunk 

 region. Compared 

 with the long trunk, the first two regions are short. 



A. The proboscis is in the shape of an acorn (hence its German 

 name " Eichel "). It can be distended and contracted, and, in the 

 limicolous manner of life, is the chief organ for burrowing. 



6 



Fio. 455.— A, Ptyehodera minuta, tVoiu the dorsal siMe ; 

 B, Balanoglossus Kowalevskii. After drawings by Peters 

 and Minot, in .SpengeVs niunograijli. 1, Proboscis; 2, collar; 

 3, branchiogenital region ; 4, hepatic region ; 5, genital folds ; 

 li, anus ; 7, branchial pores. 



