196 ZOOLOGY. 



to or beyond tlie edge of the gaping shells ; alimentary canal with the mouth 

 opening between the arms, an oesophagus, stomach with a licer-mass on 

 each side, and a short intestine ending in a blind sac, and with the stomach 

 freely suspended in the perivisceral space-. Nervous system consisting of a 

 ganglion above and beneath the oesophagus, and two lateral ventral icidely- 

 separated threads. No eyes in the adult, but present in tlie larvcv of 

 Argiope and Thecidium ; auditory sacs present in Lingula. No circulatory 

 system, the blood impelled through the visceral interspaces and tentacular 

 sinuses by ciliary action alone. Respiration carried on mainly in the 

 mantle and also in the arms. The sexes distinct. One or two pairs of seg 

 mental organs. The germ passes through a morula and gastrula stage, 

 becoming a segmented ciliated larva like that of the true worms, which be- 

 comes sessile, the shell developing from the thoracic segment of the larva and 

 gradually enclosing the animal. 



Order 1. Testicardines. Shell calcareous, of prismatic structure, with 

 hinge and teeth. Intestine ending in a blind sac. Mantlo 

 lobes united behind. (Families Terebratulidae, Spirifcridae, 

 Rhynchonellidae, Strophomenida3, and Productida?.) 



Order 2. Ecardines. Shell without a hinge ; arms without a calcare- 

 ous loop and with no median lobe. Intestine either end- 

 ing in a blind sac or with a vent opening anteriorly on the 

 right side. (Families Craniidse, Discinidae, and Lingulidge.) 



Laboratory Work. Specimens of Terebratulina can be dredged in 

 from ten to fifty fathoms on a rocky bottom north of Cape Cod, and the 

 anatomy and development of the young be studied in living examples. 

 Lingulae can be obtained at Beaufort, N". C. , and in the sandy shores 

 southward. While the general structure can be studied from alcoholic 

 specimens, the recent discoveries which have advanced our knowledge 

 of the Brachiopods have been made on living, active individuals. The 

 eggs are minute and have to be studied with high powers of the micro- 

 scope. 



CLASS VI. TUNIC AT A (Ascidians, Sea-squirts). 



General Characters of Tunicates. While the Tunicates 

 are an aberrant type of worms, with affinities on the one 

 hand to the larvae of the lower worms, -i.e., the Ccrcaria 

 form of the Trematodes, on the other hand they have a 

 startling resemblance in the tadpole-like young of many 

 genera to the vertebrate Amphioxun. The position given to 

 them between the Brachiopods and true worms not far from 



