PLATE I. 



1-16. Development of Pycnogonida. Figures from Anton Dohrn, P. P. C. Hoek, and George 



Hodge. 



a b. Abdomen. r. Proboscis. 



n. Nerve-ganglion. o. Spine on first segment of first pair of appendages. 



oc. Eye. 3- Rectum. 



The Roman numerals indicate tlie appendages in their consecutive order. 



I, 3-14, from Dolirn, Untcrsuchungen iiher Eau unci Entwicklung der Arthropoden. 2. Ueber Entwicklung 



uud Bau der Pycuogonideu. Jenaische Zeltschr., V., Taf. V., VI., 1870. 



1. Egg of Pijaiogonum littoralc after cleavage. Tlie nuclei are seen within the cleavage-spheres. Double egg- 



membrane. 



2. Section of segmented egg of Kymphon brcvicaudatnm, hardened in absolute alcohol, and colored with picro- 



carmine. The protoplasm and deutoplasm have not yet separated, and the cleavage is total. Each 

 segment has a nucleus. The faint lines within the segments denote the yolk particles, which appear as if 

 vesicular, an appearance perhaps caused by the action of the alcohol. The egg is furnished with a distinct 

 but very thin membrane. From Hoek, Report on the Pycnogonida of the Challenger Expedition, PI. XIX. 

 fig. 3, London, Edinburgh, and Dublin, 1881. 



3. Egg of Pycnogonum litlorale at a later stage than fig. 1. I, II, III, rudiments of anterior three pairs of 



appendages of embryo. 



4. Embryo of the same, later stage, r, proboscis. 



5. The same, later stage, profile view, o c, eye. a, spine arising from base of first pair of appendages. 



6. Nearly fully developed embryo of the same, from ventral side. 



7. Hatched larva of the same, ventral view. The protonymphon stage of Hoek. The anterior pair of append- 



ages is chelate, and the two following pairs are furnished with a sharp terminal claw. The spine on the 

 first pair of appendages emits a byssus-like thread secreted by a gland in the proximal segment of the 

 appendage. This thread probably serves to fasten the larva to the ovigerous legs of the adult. 



8. Eye of larva of Adielia Icciiis. 



9. Intermediate stage between larva and adult of Achclia Icevis. The three pairs of appendages of the larva have 



become much reduced, especially the second and third. The spine (a) on the mandible is disappearing. 

 The fourth and fifth pairs of appendages (IV, V) are well developed, and behind them are seen the rudi- 

 ments of the two rem.aining pairs (VI, VII) as lateral outgrowths of the body. The mouth at the end of 

 the proboscis leads into an cesophagus provided with masticating apparatus (seen at the base of the pro- 

 boscis in the figure). The intestine sending diverticula into the appendages is represented by heavy 

 shading. $, rectum, a b, abdomen. «! - a*, first to fourth sub-oesophageal nerve-ganglia. 

 If). The same, older. The proboscis has increased in size. The spine on the mandible has disappeared. The 

 second pair of appendages has lost its claw. The third pair is reduced to a short stump, which develops 

 again in the male into the ovigerous or accessory appendages. The sixth and seventh pairs have attained 

 their complete form. «•', fifth sub-cesophageal nerve-ganglion. 



II. Second appendage (palpus) of adult male Achelia IcEVis. 

 I'i. Third appendage (ovigerous or accessor}') of the same. 



13. Second appendage of female. 



14. Third appendage of the same. 



15. Adult Achc'ia Imvis, dorsal view. At the base of the palpi is seen the oculiferous tubercle bearing the eyes. 



From Hodge, List of tlie British Pycnogonoidea, with Descriptions of several new Species. Ann. Mag. 

 Nat. Hist., [3.] XIII., PI. XIII. fig. 12, 1864. 



16. Adult A'ymphon yracile. The three pairs of larval appendages are now represented by I (antennae, pedipalpi, 



or mandibles, of authors), II (palpi), and 111 (ovigerous or accessory appendages). The anterior pair are 

 innervated from the supra-a;sophageal ganglion, and may he homologous with the antenna; of Crustacea. 

 The second and following pairs receive nerves from the sub-cesophageal ganglia. The third pair of append- 

 ages serves, in the male, to carry the eggs. The nervous system is represented by the dotted lines. From 

 Hoek, Ueber Pj'cnogoiiiden. Niederlandisches Arch. Zool., III., Taf. XVI. fig. 18, 1877. 



