80 



NATURAL HISTORY OF ARTHROPODS. 



and other hydroids, as well as sponges. The dredge frequently brings up specimens. 

 About thirty species are known from American waters. 



About a hundred and fifty species of pycnogonids have been described, divided 

 into about thirty genera and four families. In the first of these families, PHOXICHILID:E, 

 the mandibles and palpi are rudimentary or absent, and the ovigerous legs are usually 

 present only in the male. The PALLEXLDJE have the mandibles well developed, the 

 palpi rudimentary or absent, while the ovigerous legs are occasionally absent in the 

 female. The COLOSSENDEID^E reverse the features of the last family, as they have the 

 mandibles rudimentary or absent, while the palpi are present. This is the largest 

 family both in genera and species, and in the size of some of its species as well, the 

 form which we figure occasionally spreading twenty-one and one half inches, while 

 Willemoes-Suhm mentions one "measuring nearly two feet across the legs." The 

 remaining family, NYMPHONID.E, has the mandibles and palpi well developed. 



On the systematic position of the Pycnogonida, anatomy and embryology throw 

 but little light. Dr. Hoek says, " The Pycnogonida form a distinct and very natural 

 group (class) of arthropodous animals. Their common progenitor (their typical 

 form) must be considered as a hyp&thetical pycnogonid with three-jointed mandibles, 

 multi-jointed palpi, and ovigerous legs with numerous rows of denticulate spines on 

 the last joints." Of their relationships to the Crustacea or the Arachnida we know 

 nothing. 



TARDIGKADA. 



The Water Bears derive their common name from a more or less fanciful resem- 

 blance to the true bears, most evident in side views, 

 while the scientific name of the group has reference to 

 their slow progression. The Tardigrada are microscopic 

 forms which are occasionally found in moist sand and 

 moss, and but rarely free in the water. The body is 

 elongated, and indistinctly shows signs of segmentation. 

 Like the sea-spiders which we have just passed, they 

 have four pairs of limbs, each terminated by two or 

 more claws. The mouth is at the anterior end of the 

 body, and is armed with organs which closely resemble 

 those of some of the true mites to be described farther 

 on. A little distance behind the mouth occurs the mus- 

 cular pharynx, and from this the alimentary tract, widen- 

 ing out in the middle to a considerable extent and then 

 contracting, pursues a straight course to the anus at the 

 posterior end of the body. No respiratory or circula- 

 tory apparatus exists. The nervous system is of the 

 usual arthropodal character, and consists of a supra- 

 oesophageal ganglion or brain, and a ventral chain of 

 four large paired ganglia corresponding to the limbs. 

 Occasionally a couple of simple eyes are connected 

 with the brain. The water-bears are hermaphroditic, 



b. Gullet, c. salivary glands. . d. piiar- the same individual and opening together into a 



ynx. e. Ovary, f. Vesicula semin- 



alis. g. Testes. 1, 2, 3, 4. Legs. cloaca. 



