1958 ] Woodland — Thermobia domestica (Packard) 
21 
shorter than in Lepisma saccharina. The author considers 
all of these characters to be secondary or specialized ones. 
Remington (personal communication, 1949) has noted sev- 
eral taxonomic characters as evidence for considering Le- 
pisma saccharina more primitive than the other two species : 
the cephalic hairs of the former occur singly and do not 
bear secondary hairs, while those of the latter occur in 
groups and bear secondary hairs; also, the former has a 
less well developed ovipositor. 
In spite of the close relationship between Thermobia do- 
mestica and Ctenolepisma lineata, each has a few specialized 
characters not shared by the other. In Ctenolepisma lineata 
a modified morula stage is absent, synchrony of cleavage is 
maintained longer, and intravitelline separation occurs late. 
In Thermobia domestica the amnio-serosal folds develop 
only weakly and there is a transverse division of the distal 
segment of the maxillary palpus. 
Although the embryology of lepismatids and symphylans 
appears superficially very different, owing to the difference 
in relative amounts of yolk in the eggs of the two groups, 
actually a great deal of embryological affinity exists between 
them. Some of the characters shared by Thermobia domes- 
tica and the symphylan Hanseniella agilis (Tiegs, 1940) are 
of particular interest: (1) The polar bodies are not ex- 
truded from the egg and degenerate rapidly. (2) A vitelline 
membrane is not apparent. (3) Periplasm has not been 
identified in freshly laid eggs. (4) The inner layer is en- 
tirely mesodermal and is produced without the formation 
of a ventral groove. (5) The time and manner of segrega- 
tion of the germ cells are similar. (6) The surface cells 
secrete a cuticle, in Hanseniella before germ band forma- 
tion, in Thermobia after germ disk formation. (7) The 
midgut epithelium is derived from yolk cells. (8) The neu- 
ropile of the nerve cord is not covered dorsally by nerve 
cells. (9) Fourteen post-cephalic segments occur in the 
adult. Anamorphosis occurs in Hanseniella but not, of 
course, in Thermobia. It may be noted, however, that 
Hanseniella hatches with 8 post-cephalic segments followed 
by a pre-anal and an anal segment. During anamorphosis 
4 more segments differentiate in front of the pre-anal seg- 
